Belowground net primary productivity and biomass allocation of a grassland in Inner Mongolia is affected by grazing intensity

The root system of permanent grasslands is of outstanding importance for resource acquisition. Particularly under semi-arid conditions, the acquisition of water and nutrients is highly variable during the vegetation growth period and between years. Additionally, grazing is repeatedly disturbing the functional equilibrium between the root system and the transpiring leaf canopy. However, very few data is available considering grazing effects on belowground net primary productivity (BNPP) and root-shoot dry mass allocation in natural grassland systems. We hypothesise that grazing significantly reduces BNPP due to carbon reallocation to shoot growth. Root biomass and BNPP were estimated by soil coring in 2004, 2005 and 2006 and from ingrowth cores in 2005 and 2006 at one site which has been protected from grazing since 1979 (UG79), at one winter grazing (WG), and one heavily grazed (HG) site. BNPP was estimated from the summation of significant increments of total and live root biomass and from accumulated root biomass of ingrowth cores. Belowground biomass varied from 1,490–2,670 g m^?2 and was significantly lower under heavy grazing than at site UG79. Root turnover varied from 0.23 to 0.33 year^?1 and was not significantly different between sites. Heavy grazing significantly decreased live root biomass and BNPP compared to site UG79. Taking BNPP estimates from live root biomass dynamics and ingrowth cores as the most reliable values, the portion of dry mass allocated belowground relative to total net primary productivity (BNPP/NPP) varied between 0.50–0.66 and was reduced under heavy grazing in 2005, but not in 2006. The positive correlation between cumulative root length density of ingrowth cores and leaf dry matter suggests that the ingrowth core method is suitable for studying BNPP in this semi-arid steppe system. Grazing effects on BNPP and BNPP/NPP should be considered in regional carbon models and estimates of belowground nutrient cycling.     

Root turnover and production by14C dilution: implications of carbon partitioning in plants

Summary Estimates of belowground net primary production (BNP) obtained by using traditional soil core harvest data are subject to a variety of potentially serious errors. In a controlled growth chamber experiment, we examined the aboveground-belowground, labile to structural tissue, and plant to soil dynamics of carbon to formulate a¹⁴C dilution technique for potential successful application in the field and to quantify sources of error in production estimates.Despite the fact that the majority of net¹⁴C movement between above- and belowground plant parts occurred between the initial labeling and day 5, significant quantities of¹⁴C were incorporated into cell-wall tissue throughout the growing period. The rate of this increase at late sampling dates was greater for roots than for shoots. Total loss of assimilated¹⁴C was 47% in wheat and 28% in blue grama. Exudation and sloughing in wheat and blue grama, respectively, was 15 and 6% of total uptake and 22 and 8% of total plant production.When root production estimates by¹⁴C dilution were corrected for the quantities of labile¹⁴C incorporated into structural carbon between two sampling dates, good agreement with actual production was found. The error associated with these estimates was ±2% compared with a range of –119 to –57% for the uncorrected estimates. Our results suggest that this technique has potential field application if sampling is performed the year after labelling.Sources of errors in harvest versus¹⁴C dilution estimates of BNP are discussed.     

Estimating Root Production: Comparison of 11 Methods in Shortgrass Steppe and Review of Biases

Estimating root production has been difficult due to multiple potential biases associated with both old and new methods. This shortgrass steppe site is the only place we are aware of that can compare most methods including sequential coring, ingrowth cores, and ingrowth donuts, ¹⁴C pulse-isotope dilution, ¹⁴C pulse-isotope turnover, rhizotron windows, and minirhizotron, and indirect methods including nitrogen budget, carbon flux, simulation carbon flow model, and regression model. We used the studies at this site, other comparisons, a summary of potential directional biases, and different ways of calculating estimates in a logical, comparative approach of evaluating methods. Much of the literature for root production is based on sequential biomass coring, a method resulting in erroneous estimates. Root ingrowth estimates of production are generally conservative compared to minirhizotron and isotope turnover methods. The size of the ingrowth area may be the most important determinant of the underestimation. Estimates based on pulse-isotope dilution are also erroneous due to non-uniform labeling of tissues. Uniform labeling is not an assumption of the pulse-isotope turnover method, and this method has the least severe potential biases. Root production estimates from pulse-isotope turnover were lower than those using minirhizotron when the most common method of calculation was used. This agrees with literature concerning bomb ¹⁴C continuous-isotope labeling comparisons with minirhizotron, although some potential biases between isotope methods are different. However, good agreement between pulse-isotope turnover and minirhizotron were obtained when minirhizotron estimates were calculated from regression of decomposition versus production to equilibrium and when pulse-isotope turnover estimates were calculated from two-phase life-span regressions. This minirhizotron method bypasses biases associated with the artificial surface similar to root-cohort methods that may be practical only in mesic systems, and takes into account both short- and long-lived roots and corrects for soil-isotope contamination that the continuous-isotope labeling bomb ¹⁴C method is not able to account for. Comparisons of these direct methods are also made with four indirect methods.     

Total Belowground Carbon Allocation in a Fast-growing Eucalyptus Plantation Estimated Using a Carbon Balance Approach

Trees allocate a large portion of gross primary production belowground for the production and maintenance of roots and mycorrhizae. The difficulty of directly measuring total belowground carbon allocation (TBCA) has limited our understanding of belowground carbon (C) cycling and the factors that control this important flux. We measured TBCA over 4 years using a conservation of mass, C balance approach in replicate stands of fast growing Eucalyptus saligna Smith with different nutrition management and tree density treatments. We measured TBCA as surface carbon dioxide (CO₂) efflux (“soil” respiration) minus C inputs from aboveground litter plus the change in C stored in roots, litter, and soil. We evaluated this C balance approach to measuring TBCA by examining (a) the variance in TBCA across replicate plots; (b) cumulative error associated with summing components to arrive at our estimates of TBCA; (c) potential sources of error in the techniques and assumptions; (d) the magnitude of changes in C stored in soil, litter, and roots compared to TBCA; and (e) the sensitivity of our measures of TBCA to differences in nutrient availability, tree density, and forest age. The C balance method gave precise estimates of TBCA and reflected differences in belowground allocation expected with manipulations of fertility and tree density. Across treatments, TBCA averaged 1.88 kg C m⁻² y⁻¹ and was 18% higher in plots planted with 10⁴ trees/ha compared to plots planted with 1111 trees/ha. TBCA was 12% lower (but not significantly so) in fertilized plots. For all treatments, TBCA declined linearly with stand age. The coefficient of variation (CV) for TBCA for replicate plots averaged 17%. Averaged across treatments and years, annual changes in C stored in soil, the litter layer, and coarse roots (−0.01, 0.06, and 0.21 kg C m⁻² y⁻¹, respectively) were small compared with surface CO₂ efflux (2.03 kg C m⁻² y⁻¹), aboveground litterfall (0.42 kg C m⁻² y⁻¹), and our estimated TBCA (1.88 kg C m⁻² y⁻¹). Based on studies from similar sites, estimates of losses of C through leaching, erosion, or storage of C in deep soil were less than 1% of annual TBCA. Received 6 March 2001; accepted 7 January 2002.     

Comprehensive assessments of root biomass and production in a Kobresia humilis meadow on the Qinghai-Tibetan Plateau

Alpine meadow covers ca. 700,000 km² with an extreme altitude range from 3200 m to 5200 m. It is the most widely distributed vegetation on the vast Qinghai-Tibetan Plateau. Previous studies suggest that meadow ecosystems play the most important role in both uptake and storage of carbon in the plateau. The ecosystem has been considered currently as an active “CO₂ sink”, in which roots may contribute a very important part, because of the large root biomass, for storage and translocation of carbon to soil. To bridge the gap between the potential importance and few experimental data, root systems, root biomass, turnover rate, and net primary production were investigated in a Kobresia humilis meadow on the plateau during the growing season from May to September in 2008 and 2009. We hypothesized that BNPP/NPP of the alpine meadow would be more than 50%, and that small diameter roots sampled in ingrowth cores have a shorter lifespan than the lager diameter roots, moreover we expected that roots in surface soils would turn over more quickly than those in deeper soil layers. The mean root mass in the 0–20 cm soil layer, investigated by the sequential coring method, was 1995?±?479 g?m^?2 and 1595?±?254 g?m^?2 in growing season of 2008 and 2009, respectively. And the mean fine root biomass in ingrowth cores of the same soil layer was 119?±?37 g?m^?2 and 196?±?45 g?m^?2 in the 2 years. Annual total NPP was 12387 kg?ha^?1?year^?1, in which 53% was allocated to roots. In addition, fine roots accounted for 33% of belowground NPP and 18% of the total NPP, respectively. Root turnover rate was 0.52 year^?1 for bulk roots and 0.74 year^?1 for fine roots. Furthermore, roots turnover was faster in surface than in deeper soil layers. The results confirmed the important role of roots in carbon storage and turnover in the alpine meadow ecosystem. It also suggested the necessity of separating fine roots from the whole root system for a better understanding of root turnover rate and its response to environmental factors.     

Belowground carbon cycling in a humid tropical forest decreases with fertilization

Only a small fraction of the carbon (C) allocated belowground by trees is retained by soils in long-lived, decay-resistant forms, yet because of the large magnitude of terrestrial primary productivity, even small changes in C allocation or retention can alter terrestrial C storage. The humid tropics exert a disproportionately large influence over terrestrial C storage, but C allocation and belowground retention in these ecosystems remain poorly quantified. Using mass balance and ¹³C isotope methods, we examined the effects of afforestation and fertilization, two land-use changes of large-scale importance, on belowground C cycling at a humid tropical site in Hawaii. Here we report that in unfertilized plots, 80% of the C allocated belowground by trees to roots and mycorrhizae was returned to the atmosphere within 1 year; 9% of the belowground C flux was retained in coarse roots and 11% was retained as new soil C. The gains in new soil C were offset entirely by losses of old soil C. Further, while fertilization early in stand development increased C storage in the litter layer and in coarse roots, it reduced by 22% the flux of C moving through roots and mycorrhizae into mineral soils. Because soil C formation rates related strongly to rhizosphere C flux, fertilization may reduce an already limited capacity of these forests to sequester decay-resistant soil C.     

Elevation effects on the carbon budget of tropical mountain forests (S Ecuador): the role of the belowground compartment

Carbon storage and sequestration in tropical mountain forests and their dependence on elevation and temperature are not well understood. In an altitudinal transect study in the South Ecuadorian Andes, we tested the hypotheses that (i) aboveground net primary production (ANPP) decreases continuously with elevation due to decreasing temperatures, whereas (ii) belowground productivity (BNPP) remains constant or even increases with elevation due to a shift from light to nutrient limitation of tree growth. In five tropical mountain forests between 1050 and 3060 m a.s.l., we investigated all major above‐ and belowground biomass and productivity components, and the stocks of soil organic carbon (SOC). Leaf biomass, stemwood mass and total aboveground biomass (AGB) decreased by 50% to 70%, ANPP by about 70% between 1050 and 3060 m, while stem wood production decreased 20‐fold. Coarse and large root biomass increased slightly, fine root biomass fourfold, while fine root production (minirhizotron study) roughly doubled between 1050 and 3060 m. The total tree biomass (above‐ and belowground) decreased from about 320 to 175 Mg dry mass ha^?1, total NPP from ca. 13.0 to 8.2 Mg ha^?1 yr^?1. The belowground/aboveground ratio of biomass and productivity increased with elevation indicating a shift from light to nutrient limitation of tree growth. We propose that, with increasing elevation, an increasing nitrogen limitation combined with decreasing temperatures causes a large reduction in stand leaf area resulting in a substantial reduction of canopy carbon gain toward the alpine tree line. We conclude that the marked decrease in tree height, AGB and ANPP with elevation in these mountain forests is caused by both a belowground shift of C allocation and a reduction in C source strength, while a temperature‐induced reduction in C sink strength (lowered meristematic activity) seems to be of secondary importance.     

Supply-side controls on soil respiration among Oregon forests

To test the hypothesis that variation in soil respiration is related to plant production across a diverse forested landscape, we compared annual soil respiration rates with net primary production and the subsequent allocation of carbon to various ecosystem pools, including leaves, fine roots, forests floor, and mineral soil for 36 independent plots arranged as three replicates of four age classes in three climatically distinct forest types. Across all plots, annual soil respiration was not correlated with aboveground net primary production (R²=0.06, P>0.1) but it was moderately correlated with belowground net primary production (R²=0.46, P<0.001). Despite the wide range in temperature and precipitation regimes experienced by these forests, all exhibited similar soil respiration per unit live fine root biomass, with about 5 g of carbon respired each year per 1 g of fine root carbon (R²=0.45, P<0.001). Annual soil respiration was only weakly correlated with dead carbon pools such as forest floor and mineral soil carbon (R²=0.14 and 0.12, respectively). Trends between soil respiration, production, and root mass among age classes within forest type were inconsistent and do not always reflect cross‐site trends. These results are consistent with a growing appreciation that soil respiration is strongly influenced by the supply of carbohydrates to roots and the rhizosphere, and that some regional patterns of soil respiration may depend more on belowground carbon allocation than the abiotic constraints imposed on subsequent metabolism.     

Above- and Belowground Net Primary Production in a Temperate Mixed Deciduous Forest

Our current ability to detect and predict changes in forest ecosystem productivity is constrained by several limitations. These include a poor understanding of belowground productivity, the short duration of most analyses, and a need for greater examination of species- or community-specific variability in productivity studies. We quantified aboveground net primary productivity (ANPP) over 3 years (1999–2001), and both belowground NPP (BNPP) and total NPP over 2 years (2000–2001) in both mesic and xeric site community types of the mixed mesophytic forest of southeastern Kentucky to examine landscape variability in productivity and its relation with soil resource [water and nitrogen (N)] availability. Across sites, ANPP was significantly correlated with N availability (R² = 0.58, P = 0.028) while BNPP was best predicted by soil moisture content (R² = 0.72, P = 0.008). Because of these offsetting patterns, total NPP was unrelated to either soil resource. Interannual variability in growing season precipitation during the study resulted in a 50% decline in mesic site litter production, possibly due to a lag effect following a moderate drought year in 1999. As a result, ANPP in mesic sites declined 27% in 2000 compared to 1999, while xeric sites had no aboveground production differences related to precipitation variability. If global climate change produces more frequent occurrences of drought, then the response of mesic sites to prolonged moisture deficiency and the consequences of shifting carbon (C) allocation on C storage will become important questions.     

ROOT PRODUCTION IN FOUR COMMUNITIES IN THE GREAT DISMAL SWAMP

A sequential coring approach was used to measure root biomass and production over 1 year in four different communities within the Great Dismal Swamp. A second method, an implanted bag technique, was also used to measure root production, and values were generally lower using this technique. On all sites, fine roots were the most dynamic root component. Both biomass (1,887 g/m²) and production (354–989 g m ² yr^-1) were highest on the mixed hardwood site, the least flooded site, and second highest on the cedar site, the site with the longest duration of soil saturation (1,033 g/m² and 274–366 g m^-2 yr^-1). The maple-gum (696 g/m² and 59–91 g m^-2 yr^-1) and cypress (824 g/m² and 68–308 g m^-2 yr^-1) sites had similarly low amounts of biomass and rates of production. Environmental parameters that influenced production include frequency and duration of flooding, and soil type. Peaks in belowground production were observed on the most productive sites (mixed hardwood and cedar) in summer and late fall-winter; the other two sites exhibited little seasonal variability. The least flooded stand appears to allocate a greater percentage of net primary production belowground than the more extensively flooded stands. The ratio of above- and belowground allocation appears to change in response to a flooding gradient. This has major implications for ecosystem functions as carbon allocation patterns determine the array of litter types generated (leaves vs. roots) which affect decomposition rates and nutrient availability.     

Partitioning pattern of carbon flux in a Kobresia grassland on the Qinghai‐Tibetan Plateau revealed by field 13C pulse‐labeling

Characterizing the carbon turnover in terrestrial ecosystems is critical for understanding and predicting carbon dynamics in ecosystems. We used in situ¹³C pulse labeling to track photosynthetic carbon fluxes from shoot to roots and to soil in a Kobresia humilis meadow on the Qinghai‐Tibet Plateau. We found that about 36.7% of labeled carbon was translocated out from the shoots within the first 24 h after photosynthetic uptake. This is equivalent to 66.1% of total ¹³C moving out from the shoot during the 32‐day chase period, indicating a rapid and large translocation of newly fixed carbon to belowground parts in these alpine plants. 58.7% of the assimilated ¹³C was transferred belowground. At the end of the chase phase, 30.9% was retained in living roots, 3.4% in dead roots, 17.2% lost as belowground respiration and 7.3% remained in the soil. In the four carbon pools (i.e., shoots, living roots, dead roots, and soil pools), living roots consistently had the highest proportion of ¹³C in the plant–soil system during the 32 days. Based on the ¹³C partitioning pattern and biomass production, we estimate a total of 4930 kg C ha^?1 was allocated belowground during the vegetation growth season in this alpine meadow. Of this, roots accumulated 2868 kg C ha^?1 and soils accumulated 613 kg C ha^?1. This study suggests that carbon storage in belowground carbon pools plays the most important role in carbon cycles in the alpine meadow.     

Root production and tissue quality in a shortgrass steppe exposed to elevated CO<Subscript>2</Subscript>: Using a new ingrowth method

A modified root ingrowth method was developed to minimize destructive sampling in experiments with limited space, and used to estimate belowground net primary production and root tissue quality in a native semiarid grassland exposed to elevated CO₂ for five years. Increases in root production of over 60% were observed with elevated CO₂ during years of intermediate levels of precipitation, with smaller effects in a very wet year and no effects in a very dry year. Aboveground to belowground production ratios, and the depth distribution of root production, did not differ between ambient and elevated CO₂ treatments. Root soluble concentrations increased an average of 11% and lignin concentrations decreased an average of 6% with elevated CO₂, while nitrogen concentrations decreased an average of 21%. However, most tissue quality responses to CO₂ varied greatly among years, and C:N ratios were higher in only one year (22 ambient vs. 33 elevated). Among years, root nitrogen concentrations declined with increasing aboveground plant nitrogen yield, and increased over the study period. Estimates of root production by the ingrowth donut method were much lower than previous estimates in the shortgrass steppe based on ¹⁴C decay. We discuss reasons why all ingrowth methods will always result in relative rather than absolute estimates of root production.     

Dynamics and dry matter production of belowground woody organs ofPinus pumila trees growing on the Kiso mountain range in central Japan

Size, biomass and spatial distribution patterns of belowground woody organs ofPinus pumila trees were investigated. Dry weight estimates of five sample trees were between 14 and 36 kg tree⁻¹. Belowground stems accounted for about 32% of the total tree weight. However, the belowground stems had extensive barkless portions, indicating that the decomposition of dead belowground stems was an important source of organic matter in the soil. The, basal diameters of adventitious roots tended to become smaller as their orginating positions neared the ground surface. It was suggested thatP. pumila trees regenerate by successively producing adventitious roots from their buried stems and moving down the slope.     

Carbon budget and methane and nitrous oxide emissions over the growing season in a Miscanthus sinensis grassland in Tomakomai,Hokkaido, Japan

Species in the Miscanthus genus have been proposed as biofuel crops that have potential to mitigate elevated atmospheric carbon dioxide (CO₂) levels and nitrous oxide (N₂O) and methane (CH₄) emissions. Miscanthus sinensis is widespread throughout Japan and has been used for biomass production for centuries. We assessed the carbon (C) budget and N₂O and CH₄ emissions over the growing season for 2 years in a M. sinensis‐dominated grassland that was naturally established around 1972 in Tomakomai, Hokkaido, Japan, which is near the northern limit for M. sinensis grassland establishment on Andisols. Average C budget was ?0.31 Mg C ha^?1, which indicates C was released from the grassland ecosystem to the atmosphere. Dominant components in the C budget appeared to be aboveground net primary production of plants (1.94–2.80 Mg C ha^?1) and heterotrophic respiration (2.27–3.11 Mg C ha^?1). The measurement of belowground net primary production (BNPP) of plants in the M. sinensis grassland was extremely variable, thus only an approximate value could be calculated. Mean C budget calculated with the approximated BNPP value was 1.47 and ?0.23 Mg C ha^?1 for 2008 and 2009, respectively. Given belowground biomass (9.46–9.86 Mg C ha^?1) was 3.1–6.5 times higher than that of aboveground biomass may provide additional evidence suggesting this grassland represents a C sink. Average CH₄ emissions across years of ?1.34 kg C ha^?1 would indicate this grassland acts as an atmospheric CH₄ sink. Furthermore, average N₂O emissions across years were 0.22 kg N ha^?1. While the site may contribute N₂O to the atmosphere, this value is lower compared with other grassland types. Global warming potential calculated with the approximated BNPP value was ?5.40 and 0.95 Mg CO₂ Eq ha^?1 for 2008 and 2009, respectively, and indicates this grassland could contribute to mitigation of global warming.     

Fine Root Production across a Primary Successional Ecosystem Chronosequence at Mt. Shasta,California

Estimating changes in belowground biomass and production is essential for understanding fundamental patterns and processes during ecosystem development. We examined patterns of fine root production, aboveground litterfall, and forest floor accumulation during forest primary succession at the Mt. Shasta Mudflows ecosystem chronosequence. Fine root production was measured using the root ingrowth cores method over 1 year, and aboveground litterfall was collected over 2 years. Fine root production increased significantly with ecosystem age, but only the youngest ecosystem was significantly different from all of the older ecosystems. Root production was 44.5 ± 13.3, 168.3 ± 20.6, 190.5 ± 33.8, and 236.3 ± 65.4 g m⁻² y⁻¹ in the 77, 255, 616, and >850-year-old ecosystems, respectively. Generally, aboveground litterfall and forest floor accumulation did not follow the same pattern as root production. The relative contribution of fine root production to total fine detrital production increased significantly with ecosystem age, from 14 to 49%, but only the youngest ecosystem was significantly different from all of the older ecosystems. Fine root production was significantly correlated with some measures of soil fertility but was not correlated with leaf or total litterfall, or forest floor accumulation. It was best predicted by soil N concentration alone, but this relationship may not be causal, as soil N concentration was also correlated with ecosystem age. For the oldest ecosystem, fine root production was also measured using the sequential intact cores/compartment-flow model method, and the difference between the two estimates was not significant. Our study suggests that the relative contribution of fine roots to fine detrital production, and hence to soil organic matter accumulation, may increase during forest primary succession.     

Linking Belowground Carbon Allocation to Anaerobic CH4 and CO2 Production in a Forested Peatland,New York State

Heterotrophic soil microorganisms rely on carbon (C) allocated belowground in plant production, but belowground C allocation (BCA) by plants is a poorly quantified part of ecosystem C cycling, especially, in peat soil. We applied a C balance approach to quantify BCA in a mixed conifer-red maple (Acer rubrum) forest on deep peat soil. Direct measurements of CH₄ and CO₂ fluxes across the soil surface (soil respiration), production of fine and small plant roots, and aboveground litterfall were used to estimate respiration by roots, by mycorrhizae and by free-living soil microorganisms. Measurements occurred in two consecutive years. Soil respiration rates averaged 1.2 bm μmol m^{? 2} s^{? 1} for CO₂ and 0.58 nmol m^{? 2} s^{? 1} for CH₄ (371 to 403 g C m^{? 2} year^{? 1}). Carbon in aboveground litter (144 g C m^{? 2} year^{? 1}) was 84% greater than C in root production (78 g C m^{? 2} year^{? 1}). Complementary in vitro assays located high rates of anaerobic microbial activity, including methanogenesis, in a dense layer of roots overlying the peat soil and in large-sized fragments within the peat matrix. Large-sized fragments were decomposing roots and aboveground leaf and twig litter, indicating that relatively fresh plant production supported most of the anaerobic microbial activity. Respiration by free-living soil microorganisms in deep peat accounted for, at most, 29 to 38 g C m^{? 2} year^{? 1}. These data emphasize the close coupling between plant production, ecosystem-level C cycling and soil microbial ecology, which BCA can help reveal.     

The dependence of root system properties on root system biomass of 10 North American grassland species

Dependence of the properties of root systems on the size of the root system may alter conclusions about differences in plant growth in different environments and among species. To determine whether important root system properties changed as root systems aged and accumulated biomass, we measured three important properties of fine roots (tissue density, diameter, and C:N) and three biomass ratios (root:shoot, fine:coarse, and shallow:deep) of monocultures of 10 North American grassland species five times during their second and third years of growth. With increasing belowground biomass, root tissue density increased and diameter decreased. This may reflect cortical loss associated with the aging of roots. For non-legumes, fine root C:N decreased with increasing root biomass, associated with decreases in soil solution NO^{3 –} concentrations. No changes in fine root C:N were detected with increasing belowground biomass for the two legumes we studied. Among all 10 species, there were generally no changes in the relative amounts of biomass in coarse and fine roots, root:shoot, or the depth placement of fine roots in the soil profile as belowground biomass increased. Though further research is needed to separate the influence of root system size, age of the roots, and changes in nutrient availability, these factors will need to be considered when comparing root functional traits among species and treatments.     

Ecosystem carbon pools,fluxes, and balances within mature tallgrass prairie restorations

Tallgrass prairie restorations can quickly accrue organic C in soil and biomass, but the rate of C accumulation diminishes through time and is highly variable among more mature prairies. Long‐term soil organic carbon (SOC) accumulation in prairies has been linked to edaphic factors such as soil texture, soil moisture, and SOC content, but it is unclear how these factors affect the ecosystem processes that are responsible for observed differences in C accumulation rates in older prairies. We measured belowground plant and SOC pools and fluxes within 27–36‐year‐old restored tallgrass prairies in order to quantify total C storage, determine the net ecosystem production of C (NEP‐C), and explore which edaphic factors influence the ecosystem processes responsible for divergent NEP‐C. We found that 11% of organic C was stored in biomass, and we estimate that one‐third of post‐restoration C sequestration has occurred in biomass, thereby highlighting biomass as a large but often overlooked C pool. Belowground biomass and soil C pools were notably smaller than those reported for remnant prairie, suggesting that future belowground C accumulation could still occur. During this study, the prairies appeared to be a net source of C, although the range of NEP‐C values encompassed zero. Sand content positively affected NEP‐C via increased belowground biomass production‐C inputs, and SOC negatively affected NEP‐C due to increased soil respiration C outputs. However, soil moisture had a smaller negative effect on soil respiration, indicating that both SOC and soil moisture play important roles in determining prairie C balance.     

As above,not so below: Long-term dynamics of net primary production across a dryland transition zone

Drylands are key contributors to interannual variation in the terrestrial carbon sink, which has been attributed primarily to broad-scale climatic anomalies that disproportionately affect net primary production (NPP) in these ecosystems. Current knowledge around the patterns and controls of NPP is based largely on measurements of aboveground net primary production (ANPP), particularly in the context of altered precipitation regimes. Limited evidence suggests belowground net primary production (BNPP), a major input to the terrestrial carbon pool, may respond differently than ANPP to precipitation, as well as other drivers of environmental change, such as nitrogen deposition and fire. Yet long-term measurements of BNPP are rare, contributing to uncertainty in carbon cycle assessments. Here, we used 16 years of annual NPP measurements to investigate responses of ANPP and BNPP to several environmental change drivers across a grassland–shrubland transition zone in the northern Chihuahuan Desert. ANPP was positively correlated with annual precipitation across this landscape; however, this relationship was weaker within sites. BNPP, on the other hand, was weakly correlated with precipitation only in Chihuahuan Desert shrubland. Although NPP generally exhibited similar trends among sites, temporal correlations between ANPP and BNPP within sites were weak. We found chronic nitrogen enrichment stimulated ANPP, whereas a one-time prescribed burn reduced ANPP for nearly a decade. Surprisingly, BNPP was largely unaffected by these factors. Together, our results suggest that BNPP is driven by a different set of controls than ANPP. Furthermore, our findings imply belowground production cannot be inferred from aboveground measurements in dryland ecosystems. Improving understanding around the patterns and controls of dryland NPP at interannual to decadal scales is fundamentally important because of their measurable impact on the global carbon cycle. This study underscores the need for more long-term measurements of BNPP to improve assessments of the terrestrial carbon sink, particularly in the context of ongoing environmental change.     

The effect of soil pH on rhizosphere carbon flow of Lolium perenne

Perennial rye-grass plants were grown at 15°C in microcosms containing soil sampled from field plots that had been maintained at constant pH for the last 30 years. Six soil pH values were tested in the experiment, with pH ranging from 4.3–6.5. After 3 weeks growth in the microcosms, plant shoots were exposed to a pulse of ¹⁴C-CO₂. The fate of this label was determined by monitoring ¹⁴C-CO₂ respired by the plant roots/soil and by the shoots. The ¹⁴C remaining in plant roots and shoots was determined when the plants were harvested 7 days after receiving the pulse label. The amount of ¹⁴C (expressed as a percentage of the total ¹⁴C fixed by the plant) lost from the plant roots increased from 12.3 to 30.6% with increasing soil pH from 4.3 to 6. Although a greater percentage of the fixed ¹⁴C was respired by the root/soil as soil pH increased, plant biomass was greater with increasing soil pH. Possible reasons for observed changes in the pattern of ¹⁴C distribution are discussed and, it is suggested that changes in the soil microbial biomass and in plant nitrogen nutrition may, in particular be key factors which led to increased loss of carbon from plant roots with increasing soil pH.