Impact of agricultural management practices on soil organic carbon: simulation of Australian wheat systems

Quantifying soil organic carbon (SOC) dynamics at a high spatial and temporal resolution in response to different agricultural management practices and environmental conditions can help identify practices that both sequester carbon in the soil and sustain agricultural productivity. Using an agricultural systems model (the Agricultural Production Systems sIMulator), we conducted a high spatial resolution and long‐term (122 years) simulation study to identify the key management practices and environmental variables influencing SOC dynamics in a continuous wheat cropping system in Australia's 96 million ha cereal‐growing regions. Agricultural practices included five nitrogen application rates (0–200 kg N ha^?1 in 50 kg N ha^?1 increments), five residue removal rates (0–100% in 25% increments), and five residue incorporation rates (0–100% in 25% increments). We found that the change in SOC during the 122‐year simulation was influenced by the management practices of residue removal (linearly negative) and fertilization (nonlinearly positive) – and the environmental variables of initial SOC content (linearly negative) and temperature (nonlinearly negative). The effects of fertilization were strongest at rates up to 50 kg N ha^?1, and the effects of temperature were strongest where mean annual temperatures exceeded 19 °C. Reducing residue removal and increasing fertilization increased SOC in most areas except Queensland where high rates of SOC decomposition caused by high temperature and soil moisture negated these benefits. Management practices were particularly effective in increasing SOC in south‐west Western Australia – an area with low initial SOC. The results can help target agricultural management practices for increasing SOC in the context of local environmental conditions, enabling farmers to contribute to climate change mitigation and sustaining agricultural production.     

Aggregation and C and N contents of soil organic matter fractions in a permanent raised-bed planting system in the Highlands of Central Mexico

Permanent raised bed planting with crop residue retention is a form of conservation agriculture that has been proposed as an alternative to conventional tillage for wheat production systems in the Central Highlands of Mexico. A field experiment comparing permanent and tilled raised beds with different residue management under rainfed conditions was started at El Batán (State of Mexico, Mexico) in 1999. The percentage of small and large macroaggregates and mean weight diameter (MWD) was significantly larger in permanent raised beds compared to conventionally tilled raised beds both with full crop residue retention (average for maize and wheat), while the percentages free microaggregates was lower. The percentages of small and large macroaggregates and mean weight diameter (MWD) was significantly larger in permanent raised beds with residue retention compared to permanent raised beds with removal of the residue (average for maize and wheat), while the percentages free microaggregates and silt and clay fraction was lower. Cultivation of maize significantly reduced the large macroaggregates, while wheat reduced the silt and clay fraction (average over all systems). Cultivation of maize reduced the C and N content of the free microaggregates compared to soil cultivated with wheat, while removal of plant residue reduced the C and N content of the silt and clay fraction compared to soil where residue was retained. The C and N content of the coarse particulate organic matter (cPOM) and microaggregates within the macroaggregates was significantly larger in permanent raised beds compared to conventionally tilled raised beds both with full residue retention, while C and N content of the cPOM was significantly lower when residue was removed or partially removed compared to the soil where the residue was retained. The δ ¹³C ‰ signatures of the macroaggregates, microaggregates, the silt and clay fraction, cPOM and microaggregates within the macroaggregates were not affected by tillage or residue management when wheat was the last crop, but removal of residue reduced the δ ¹³C ‰ signatures of the macro-, microaggregates and microaggregates within the macroaggregates significantly compared to soil where the residue was retained. Retaining only 30–50% of the organic residue still improved the soil structure considerably compared to plots where it was removed completely. Permanent raised beds without residue retention, however, is a practice leading to soil degradation. Kelly Lichter and Bram Govaerts contributed equally to this publication.     

Land use change from C3 grassland to C4 Miscanthus: effects on soil carbon content and estimated mitigation benefit after six years

To date, most Miscanthus trials and commercial fields have been planted on arable land. Energy crops will need to be grown more on lower grade lands unsuitable for arable crops. Grasslands represent a major land resource for energy crops. In grasslands, where soil organic carbon (SOC) levels can be high, there have been concerns that the carbon mitigation benefits of bioenergy from Miscanthus could be offset by losses in SOC associated with land use change. At a site in Wales (UK), we quantified the relatively short‐term impacts (6 years) of four novel Miscanthus hybrids and Miscanthus × giganteus on SOC in improved grassland. After 6 years, using stable carbon isotope ratios (¹³C/¹²C), the amount of Miscanthus derived C (C4) in total SOC was considerable (ca. 12%) and positively correlated to belowground biomass of different hybrids. Nevertheless, significant changes in SOC stocks (0–30 cm) were not detected as C4 Miscanthus carbon replaced the initial C3 grassland carbon; however, initial SOC decreased more in the presence of higher belowground biomass. We ascribed this apparently contradictory result to the rhizosphere priming effect triggered by easily available C sources. Observed changes in SOC partitioning were modelled using the RothC soil carbon turnover model and projected for 20 years showing that there is no significant change in SOC throughout the anticipated life of a Miscanthus crop. We interpret our observations to mean that the new labile C from Miscanthus has replaced the labile C from the grassland and, therefore, planting Miscanthus causes an insignificant change in soil organic carbon. The overall C mitigation benefit is therefore not decreased by depletion of soil C and is due to substitution of fossil fuel by the aboveground biomass, in this instance 73–108 Mg C ha^?1 for the lowest and highest yielding hybrids, respectively, after 6 years.     

Carbon sequestration in soil in a semi‐natural Miscanthus sinensis grassland and Cryptomeria japonica forest plantation in Aso,Kumamoto, Japan

Although Miscanthus sinensis grasslands (Misc‐GL) and Cryptomeria japonica forest plantations (Cryp‐FP) are proposed bioenergy feedstock systems, their relative capacity to sequester C may be an important factor in determining their potential for sustainable bioenergy production. Therefore, our objective was to quantify changes in soil C sequestration 47 years after a Misc‐GL was converted to a Cryp‐FP. The study was conducted on adjacent Misc‐GL and Cryp‐FP located on Mt. Aso, Kumamoto, Japan. After Cryp‐FP establishment, only the Misc‐GL continued to be managed by annual burning every March. Mass C and N, δ¹³C, and δ¹⁵N at 0–30 cm depth were measured in 5 cm increments. Carbon and N concentrations, C:N ratio, δ¹³C, and δ¹⁵N were measured in litter and/or ash, and rhizomes or roots. Although C input in Misc‐GL by M. sinensis was approximately 36% of that in Cryp‐FP by C. japonica, mean annual soil C sequestration in Misc‐GL (503 kg C ha^?1 yr^?1) was higher than that in Cryp‐FP (284 kg C ha^?1 yr^?1). This was likely the result of larger C input from aboveground litter to soil, C‐quality (C:N ratio and lignin concentration in aboveground litter) and possibly more recalcitrant C (charcoal) inputs by annual burning. The difference in soil δ¹⁵N between sites indicated that organic C with N had greater cycling between heterotrophic microbes and soil and produces more recalcitrant humus in Misc‐GL than in Cryp‐FP. Our data indicate that in terms of soil C sequestration, maintenance of Misc‐GL may be more advantageous than conversion to Cryp‐FP in Aso, Japan.     

Aboveground productivity and soil carbon storage of biofuel crops in Ohio

Biofuel crops may help achieve the goals of energy‐efficient renewable ethanol production and greenhouse gas (GHG) mitigation through carbon (C) storage. The objective of this study was to compare the aboveground biomass yields and soil organic C (SOC) stocks under four crops (no‐till corn, switchgrass, indiangrass, and willow) 7 years since establishment at three sites in Ohio to determine if high‐yielding biofuel crops are also capable of high levels of C storage. Corn grain had the highest potential ethanol yields, with an average of more than 4100 L ha^?1, and ethanol yields increased if both corn grain and stover were converted to biofuel, while willow had the lowest yields. The SOC concentration in soils under biofuels was generally unaffected by crop type; at one site, soil in the top 10 cm under willow contained nearly 13 Mg C ha^?1 more SOC (or 29% more) than did soils under switchgrass or corn. Crop type affected SOC content of macroaggregates in the top 10 cm of soil, where macroaggregates in soil under corn had lower C, N and C : N ratios than those under perennial grasses or trees. Overall, the results suggest that no‐till corn is capable of high ethanol yields and equivalent SOC stocks to 40 cm depth. Long‐term monitoring and measurement of SOC stocks at depth are required to determine whether this trend remains. In addition, ecological, energy, and GHG assessments should be made to estimate the C footprint of each feedstock.     

N fertilizer and harvest impacts on bioenergy crop contributions to SOC

Belowground root biomass is infrequently measured and simply represented in models that predict landscape‐level changes to soil carbon stocks and greenhouse gas balances. Yet, crop‐specific responses to N fertilizer and harvest treatments are known to impact both plant allocation and tissue chemistry, potentially altering decomposition rates and the direction and magnitude of soil C stock changes and greenhouse gas fluxes. We examined switchgrass (Panicum virgatum L.) and corn (Zea mays L.,) yields, belowground root biomass, C, N and soil particulate organic matter‐C (POM‐C) in a 9‐year rainfed study of N fertilizer rate (0, 60, 120 and 180 kg N ha^?1) and harvest management near Mead, NE, USA. Switchgrass was harvested with one pass in either August or postfrost, and for no‐till (NT) corn, either 50% or no stover was removed. Switchgrass had greater belowground root biomass C and N (6.39, 0.10 Mg ha^?1) throughout the soil profile compared to NT‐corn (1.30, 0.06 Mg ha^?1) and a higher belowground root biomass C:N ratio, indicating greater recalcitrant belowground root biomass C input beneath switchgrass. There was little difference between the two crops in soil POM‐C indicating substantially slower decomposition and incorporation into SOC under switchgrass, despite much greater root C. The highest N rate decreased POM‐C under both NT‐corn and switchgrass, indicating faster decomposition rates with added fertilizer. Residue removal reduced corn belowground root biomass C by 37% and N by 48% and subsequently reduced POM‐C by 22% compared to no‐residue removal. Developing productive bioenergy systems that also conserve the soil resource will require balancing fertilization that maximizes aboveground productivity but potentially reduces SOC sequestration by reducing belowground root biomass and increasing root and soil C decomposition.     

Contribution of N from green harvest residues for sugarcane nutrition in Brazil

Brazil is recognized as a prominent renewable energy producer due to the production of ethanol from sugarcane. However, in order for this source of energy to be considered truly sustainable, conservation management practices, such as harvesting the cane green (without burning) and retaining the trash in the field, need to be adopted. This management practice affects mostly the nitrogen (N) cycle through the effect of trash on immobilization–mineralization of N by soil microorganisms. The aim of the experiments reported here was to evaluate N recovery from trash (trash‐N) by sugarcane during three ratoon crop seasons: 2007, 2008 and 2009. Two field experiments were carried out, one in Jaboticabal and the other in Pradopolis, in the state of Sao Paulo, Brazil. The experiments were set up in a randomized block design with four replications. Within each plot, microplots were installed where the original trash was replaced by trash labelled with ¹⁵N, and maintained up to the fourth crop cycle. Trash‐N recovery was higher in the Jaboticabal site, the most productive one, than in the Pradópolis site. The average trash‐N recovery across the two sites after three crop cycles was 7.6 kg ha^?1 (or 16.2% of the initial N content in trash), with the remaining trash‐N being incorporated into soil organic matter reserves. While these results indicate that the value of trash for sugarcane nutrition is limited in the short term, maintaining trash on the field will serve as a long‐term source of N and C for the soil.     

Miscanthus × giganteus leaf senescence,decomposition and C and N inputs to soil

Energy crops are currently promoted as potential sources of alternative energy that can help mitigate the climate change caused by greenhouse gases (GHGs). The perennial crop Miscanthus × giganteus is considered promising due to its high potential for biomass production under conditions of low input. However, to assess its potential for GHG mitigation, a better quantification of the crop's contribution to soil organic matter recycling under various management systems is needed. The aim of this work was to study the effect of abscised leaves on carbon (C) and nitrogen (N) recycling in a Miscanthus plantation. The dynamics of senescent leaf fall, the rate of leaf decomposition (using a litter bag approach) and the leaf accumulation at the soil surface were tracked over two 1‐year periods under field conditions in Northern France. The fallen leaves represented an average yearly input of 1.40 Mg C ha^?1 and 16 kg N ha^?1. The abscised leaves lost approximately 54% of their initial mass in 1 year due to decomposition; the remaining mass, accumulated as a mulch layer at the soil surface, was equivalent to 7 Mg dry matter (DM) ha^?1 5 years after planting. Based on the estimated annual leaf‐C recycling rate and a stabilization rate of 35% of the added C, the annual contribution of the senescent leaves to the soil C was estimated to be approximately 0.50 Mg C ha^?1yr^?1 or 10 Mg C ha^?1 total over the 20‐year lifespan of a Miscanthus crop. This finding suggested that for Miscanthus, the abscised leaves contribute more to the soil C accumulation than do the rhizomes or roots. In contrast, the recycling of the leaf N to the soil was less than for the other N fluxes, particularly for those involving the transfer of N from the tops of the plant to the rhizome.     

Conventionally Tilled and Permanent Raised Beds with Different Crop Residue Management: Effects on Soil C and N Dynamics

Conservation tillage in its version of permanent bed planting under zero-tillage with crop residue retention has been proposed as an alternative wheat production system for northwest Mexico. However, little is known about the dynamics of C and N in soils under wheat/maize on permanent beds (PB) where straw was burned, removed, partly removed or retained, as opposed to conventionally tilled beds (CTB) where straw was incorporated. We investigated the dynamics of soil C and N and normalized difference vegetative index (NDVI) crop values in zero-tilled PB and CTB after 26 successive maize and wheat crops. Organic C and total N were respectively, 1.15 and 1.17 times greater in PB with straw partly removed and with straw retained on the surface, than in CTB with straw incorporated. Organic C and total N were 1.10 times greater in soils with 300 kg N ha⁻¹ added than in unfertilized soil. Cumulative production of CO₂ was lower under CTB with straw incorporated than under PB treatments, and CO₂ production increased with increments in inorganic fertilizer. The N-mineralization rate was 1.18 times greater than in unamended soils when 150 kg inorganic N ha⁻¹ was applied, and 1.48 times greater when 300 kg inorganic N ha⁻¹ was added. The N-mineralization rate was significantly (1.66 times) greater in PB where the straw was burned or retained on the surface than in CTB where the straw was incorporated, but significantly (1.25 times) lower than in PB with straw partly removed. The NDVI values reached a maximum 56 days after planting and decreased thereafter. The NDVI for unfertilized soil were similar for CTB with straw incorporated, PB with straw partly removed, and PB with straw retained on the surface, but significantly lower for PB with straw burned and PB with straw removed. In soils to which 150 or 300 kg N ha⁻¹ was added, NDVI was significantly lower for PB with straw burned than for other treatments. Among other things, this suggests the utility of rotating maize or wheat with crops whose residues have lower C–N ratios, thus avoiding immobilization of large amounts of N for extended periods. PB with residue burning, however, is an unsustainable practice leading to low crop performance and soil and environmental degradation.     

Cover crop root contributions to soil carbon in a no‐till corn bioenergy cropping system

Crop residues are potential biofuel feedstocks, but residue removal may reduce soil carbon (C). The inclusion of a cover crop in a corn bioenergy system could provide additional biomass, mitigating the negative effects of residue removal by adding to stable soil C pools. In a no‐till continuous corn bioenergy system in the northern US Corn Belt, we used ¹³CO₂ pulse labeling to trace plant C from a winter rye (Secale cereale) cover crop into different soil C pools for 2 years following rye cover crop termination. Corn stover left as residue (30% of total stover) contributed 66, corn roots 57, rye shoots 61, rye roots 50, and rye rhizodeposits 25 g C m^?2 to soil. Five months following cover crop termination, belowground cover crop inputs were three times more likely to remain in soil C pools than were aboveground inputs, and much of the root‐derived C was in mineral‐associated soil fractions. After 2 years, both above‐ and belowground inputs had declined substantially, indicating that the majority of both root and shoot inputs are eventually mineralized. Our results underscore the importance of cover crop roots vs. shoots and the importance of cover crop rhizodeposition (33% of total belowground cover crop C inputs) as a source of soil C. However, the eventual loss of most cover crop C from these soils indicates that cover crops will likely need to be included every year in rotations to accumulate soil C.     

Measuring carbon and N2 fixation in field populations of colonial and free‐living unicellular cyanobacteria using nanometer‐scale secondary ion mass spectrometry1

Unicellular cyanobacteria are now recognized as important to the marine N and C cycles in open ocean gyres, yet there are few direct in situ measurements of their activities. Using a high‐resolution nanometer scale secondary ion mass spectrometer (nanoSIMS), single cell N₂ and C fixation rates were estimated for unicellular cyanobacteria resembling N₂ fixer Crocosphaera watsonii. Crocosphaera watsonii‐like cells were observed in the subtropical North Pacific gyre (22°45′ N, 158°0′ W) as 2 different phenotypes: colonial and free‐living. Colonies containing 3–242 cells per colony were observed and cell density in colonies increased with incubation time. Estimated C fixation rates were similarly high in both phenotypes and unexpectedly for unicellular cyanobacteria 85% of the colonial cells incubated during midday were also enriched in ¹⁵N above natural abundance. Highest ¹⁵N enrichment and N₂ fixation rates were found in cells incubated overnight where up to 64% of the total daily fixed N in the upper surface waters was attributed to both phenotypes. The colonial cells retained newly fixed C in a sulfur‐rich matrix surrounding the cells and often cells of both phenotypes possessed areas (<1 nm) of enriched ¹⁵N and ¹³C resembling storage granules. The nanoSIMS imaging of the colonial cells also showed evidence for a division of N₂ and C fixation activity across the colony where few individual cells (<34%) in a given colony were enriched in both ¹⁵N and ¹³C above the colony average. Our results provide new insights into the ecophysiology of unicellular cyanobacteria.     

Mycorrhizal mediation of plant N acquisition and residue decomposition: Impact of mineral N inputs

Mycorrhizas are ubiquitous plant–fungus mutualists in terrestrial ecosystems and play important roles in plant resource capture and nutrient cycling. Sporadic evidence suggests that anthropogenic nitrogen (N) input may impact the development and the functioning of arbuscular mycorrhizal (AM) fungi, potentially altering host plant growth and soil carbon (C) dynamics. In this study, we examined how mineral N inputs affected mycorrhizal mediation of plant N acquisition and residue decomposition in a microcosm system. Each microcosm unit was separated into HOST and TEST compartments by a replaceable mesh screen that either prevented or allowed AM fungal hyphae but not plant roots to grow into the TEST compartments. Wild oat (Avena fatua L.) was planted in the HOST compartments that had been inoculated with either a single species of AM fungus, Glomus etunicatum, or a mixture of AM fungi including G. etunicatum. Mycorrhizal contributions to plant N acquisition and residue decomposition were directly assessed by introducing a mineral ¹⁵N tracer and ¹³C‐rich residues of a C₄ plant to the TEST compartments. Results from ¹⁵N tracer measurements showed that AM fungal hyphae directly transported N from the TEST soil to the host plant. Compared with the control with no penetration of AM fungal hyphae, AM hyphal penetration led to a 125% increase in biomass ¹⁵N of host plants and a 20% reduction in extractable inorganic N in the TEST soil. Mineral N inputs to the HOST compartments (equivalent to 5.0 g N m^?2 yr^?1) increased oat biomass and total root length colonized by mycorrhizal fungi by 189% and 285%, respectively, as compared with the no‐N control. Mineral N inputs to the HOST plants also reduced extractable inorganic N and particulate residue C proportion by 58% and 12%, respectively, in the corresponding TEST soils as compared to the no‐N control, by stimulating AM fungal growth and activities. The species mixture of mycorrhizal fungi was more effective in facilitating N transport and residue decomposition than the single AM species. These findings indicate that low‐level mineral N inputs may significantly enhance nutrient cycling and plant resource capture in terrestrial ecosystems via stimulation of root growth, mycorrhizal functioning, and residue decomposition. The long‐term effects of these observed alterations on soil C dynamics remain to be investigated.     

Carbon footprint of rice production under biochar amendment – a case study in a Chinese rice cropping system

As a controversial strategy to mitigate global warming, biochar application into soil highlights the need for life cycle assessment before large‐scale practice. This study focused on the effect of biochar on carbon footprint of rice production. A field experiment was performed with three treatments: no residue amendment (Control), 6 t ha^?1 yr^?1 corn straw (CS) amendment, and 2.4 t ha^?1 yr^?1 corn straw‐derived biochar amendment (CBC). Carbon footprint was calculated by considering carbon source processes (pyrolysis energy cost, fertilizer and pesticide input, farmwork, and soil greenhouse gas emissions) and carbon sink processes (soil carbon increment and energy offset from pyrolytic gas). On average over three consecutive rice‐growing cycles from year 2011 to 2013, the CS treatment had a much higher carbon intensity of rice (0.68 kg CO₂‐C equivalent (CO₂‐C_e) kg^?1 grain) than that of Control (0.24 kg CO₂‐C_e kg^?1 grain), resulting from large soil CH₄ emissions. Biochar amendment significantly increased soil carbon pool and showed no significant effect on soil total N₂O and CH₄ emissions relative to Control; however, due to a variation in net electric energy input of biochar production based on different pyrolysis settings, carbon intensity of rice under CBC treatment ranged from 0.04 to 0.44 kg CO₂‐C_e kg^?1 grain. The results indicated that biochar strategy had the potential to significantly reduce the carbon footprint of crop production, but the energy‐efficient pyrolysis technique does matter.     

Soil warming alters microbial substrate use in alpine soils

Will warming lead to an increased use of older soil organic carbon (SOC) by microbial communities, thereby inducing C losses from C‐rich alpine soils? We studied soil microbial community composition, activity, and substrate use after 3 and 4 years of soil warming (+4 °C, 2007–2010) at the alpine treeline in Switzerland. The warming experiment was nested in a free air CO₂ enrichment experiment using depleted ¹³CO₂ (δ¹³C = ?30‰, 2001–2009). We traced this depleted ¹³C label in phospholipid fatty acids (PLFA) of the organic layer (0–5 cm soil depth) and in C mineralized from root‐free soils to distinguish substrate ages used by soil microorganisms: fixed before 2001 (‘old’), from 2001 to 2009 (‘new’) or in 2010 (‘recent’). Warming induced a sustained stimulation of soil respiration (+38%) without decline in mineralizable SOC. PLFA concentrations did not reveal changes in microbial community composition due to soil warming, but soil microbial metabolic activity was stimulated (+66%). Warming decreased the amount of new and recent C in the fungal biomarker 18:2ω6,9 and the amount of new C mineralized from root‐free soils, implying a shift in microbial substrate use toward a greater use of old SOC. This shift in substrate use could indicate an imbalance between C inputs and outputs, which could eventually decrease SOC storage in this alpine ecosystem.     

Nitrogen and harvest effects on soil properties under rainfed switchgrass and no‐till corn over 9 years: implications for soil quality

Nitrogen fertilizer and harvest management will alter soils under bioenergy crop production and the long‐term effects of harvest timing and residue removal remain relatively unknown. Compared to no‐tilled corn (NT‐C, Zea mays L.), switchgrass (Panicum virgatum L.) is predicted to improve soil properties [i.e. soil organic C (SOC), soil microbial biomass (SMB‐C), and soil aggregation] due to its perennial nature and deep‐rooted growth form, but few explicit field comparisons exist. We assessed soil properties over 9 years for a rainfed study of N fertilizer rate (0, 60, 120, and 180 kg N ha^?1) and harvest management on switchgrass (harvested in August and postfrost) and NT‐C (with and without 50% stover removal) in eastern NE. We measured SOC, aggregate stability, SMB‐C, bulk density (BD), pH, P and K in the top 0–30 cm. Both NT‐C and switchgrass increased SMB‐C, SOC content, and aggregate stability over the 9 years, reflecting improvement from previous conventional management. However, the soils under switchgrass had double the percent aggregate stability, 1.3 times more microbial biomass, and a 5–8% decrease in bulk density in the 0–5 and 5–10 cm depths compared to NT‐C. After 9 years, cumulative decrease in available P was significantly greater beneath NT‐C (?24.0 kg P ha^?1) compared to switchgrass (?5.4 kg P ha^?1). When all measured soil parameters were included in the Soil Management Assessment Framework (SMAF), switchgrass improved soil quality index over time (ΔSQI) in all depths. NT‐C without residue removal did not affect ΔSQI, but 50% residue removal decreased ΔSQI (0–30 cm) due to reduced aggregate stability and SMB‐C. Even with best‐management practices such as NT, corn stover removal will have to be carefully managed to prevent soil degradation. Long‐term N and harvest management studies that include biological, chemical, and physical soil measurements are necessary to accurately assess bioenergy impacts on soils.     

A long‐term nitrogen fertilizer gradient has little effect on soil organic matter in a high‐intensity maize production system

Global maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affecting SOC within maize‐based systems due to its effects on crop residue production and SOC mineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0–269 kg N ha^?1 yr^?1) that created a large range in crop residue inputs (3.60–9.94 Mg dry matter ha^?1 yr^?1), we provide the first agronomic assessment of long‐term N fertilizer effects on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in physico‐chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra‐aggregate POM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition of POM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha^?1 yr^?1) and an excessive N rate (269 kg N ha^?1 yr^?1), there were no differences between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N₂O and NO₃ emissions exponentially increase when agronomic optimum N is surpassed.     

Exploring soil microbial 16S rRNA sequence data to increase carbon yield and nitrogen efficiency of a bioenergy crop

Crop residues returned to the soil are important for the preservation of soil quality, health, and biodiversity, and they increase agriculture sustainability by recycling nutrients. Sugarcane is a bioenergy crop that produces huge amounts of straw (also known as trash) every year. In addition to straw, the ethanol industry also generates large volumes of vinasse, a liquid residue of ethanol production, which is recycled in sugarcane fields as fertilizer. However, both straw and vinasse have an impact on N₂O fluxes from the soil. Nitrous oxide is a greenhouse gas that is a primary concern in biofuel sustainability. Because bacteria and archaea are the main drivers of N redox processes in soil, in this study we propose the identification of taxa related with N₂O fluxes by combining functional responses (N₂O release) and the abundance of these microorganisms in soil. Using a large‐scale in situ experiment with ten treatments, an intensive gas monitoring approach, high‐throughput sequencing of soil microbial 16S rRNA gene and powerful statistical methods, we identified microbes related to N₂O fluxes in soil with sugarcane crops. In addition to the classical denitrifiers, we identified taxa within the phylum Firmicutes and mostly uncharacterized taxa recently described as important drivers of N₂O consumption. Treatments with straw and vinasse also allowed the identification of taxa with potential biotechnological properties that might improve the sustainability of bioethanol by increasing C yields and improving N efficiency in sugarcane fields.     

Changes in soil carbon stocks under perennial and annual bioenergy crops

Bioenergy crops are expected to provide biomass to replace fossil resources and reduce greenhouse gas emissions. In this context, changes in soil organic carbon (SOC) stocks are of primary importance. The aim of this study was to measure changes in SOC stocks in bioenergy cropping systems comparing perennial (Miscanthus × giganteus and switchgrass), semi‐perennial (fescue and alfalfa), and annual (sorghum and triticale) crops, all established after arable crops. The soil was sampled at the start of the experiment and 5 or 6 years later. SOC stocks were calculated at equivalent soil mass, and δ¹³C measurements were used to calculate changes in new and old SOC stocks. Crop residues found in soil at the time of SOC measurements represented 3.5–7.2 t C ha^?1 under perennial crops vs. 0.1–0.6 t C ha^?1 for the other crops. During the 5‐year period, SOC concentrations under perennial crops increased in the surface layer (0–5 cm) and slightly declined in the lower layers. Changes in δ¹³C showed that C inputs were mainly located in the 0–18 cm layer. In contrast, SOC concentrations increased over time under semi‐perennial crops throughout the old ploughed layer (ca. 0–33 cm). SOC stocks in the old ploughed layer increased significantly over time under semi‐perennials with a mean increase of 0.93 ± 0.28 t C ha^?1 yr^?1, whereas no change occurred under perennial or annual crops. New SOC accumulation was higher for semi‐perennial than for perennial crops (1.50 vs. 0.58 t C ha^?1 yr^?1, respectively), indicating that the SOC change was due to a variation in C input rather than a change in mineralization rate. Nitrogen fertilization rate had no significant effect on SOC stocks. This study highlights the interest of comparing SOC changes over time for various cropping systems.     

鼎湖山3种演替阶段森林土壤C、N、P现状及动态

为探讨森林演替过程中土壤C、N、P的变化,通过测定鼎湖山3种演替阶段的森林土壤有机碳(SOC)、总氮(TN)、总磷(TP)含量,对他们的化学计量进行分析。结果表明,鼎湖山3种森林土壤SOC和TN随演替阶段而增加,演替中后期表层土壤(0~20 cm)与演替初期的差异达到显著水平(P0.05),在土壤剖面上的分布都呈现显著的表层富集现象,且表层土壤与其他土层均有显著差异(P0.05)。土壤TP含量随演替阶段没有呈现出有规律的变化,不同演替阶段间也没有显著差异,但不同演替阶段土壤TP在土壤剖面上的分布表现不同,演替前期土壤TP含量随着土层深度增加而增加,演替后期土壤TP随土层深度的增加而降低,而演替中期土壤TP含量在各土层间没有显著差异。土壤C∶N不受土层深度和演替进程的影响,而土壤C∶P和N∶P均表现为随演替阶段而增加,随土层加深而降低。这些揭示了森林土壤SOC、TN和TP含量随演替进展及其在土壤剖面上的分布取决于土壤C、N、P的来源方式。