长白山原始阔叶红松林土壤有机质组分小尺度空间异质性

土壤有机质(SOM)对于维持生态系统生产力具有非常重要的意义,有机质的组成、空间分布和空间关联性是影响和控制诸多生态系统过程的重要因素。应用地统计学方法,对长白山原始阔叶红松林局部尺度内0—20 cm土壤有机质与活性有机质的空间异质性进行了研究,并通过交叉半方差分析探讨了二者之间的相关性。研究结果表明:(1)总体上来说,土壤有机碳(SOC)、全氮(TN)、颗粒态有机碳(POC)和颗粒态有机氮(PON)空间异质性较小;而土壤微生物量碳(MBC)、微生物量氮(MBN)和表层(0—10 cm)溶解性有机碳(DOC)的空间异质性较大;(2)SOC、TN、MBC、DOC、POC和PON随着深度的增加空间自相关性增加;而溶解性有机氮(DON)的空间自相关性随深度的增加变化不大;(3)SOC与TN在表层和下层(10—20 cm)均存在空间上的正相关关系;(4)SOC、TN在表层和下层分别与MBC、MBN、DOC、DON和POC呈空间上的正相关性,但是与PON之间的空间相关关系较差;(5)不同土层深度的土壤活性有机质之间的相关关系存在差异。在表层,除POC,PON外,其余土壤活性有机质组分在空间上两两相关;但是随着土壤深度的增加,活性有机质变量之间在空间上两两相关。研究结果表明土壤有机质组分在长白山原始阔叶红松林小尺度内存在不同空间异质性和空间关联性,这为人们更好的理解森林生态系统功能(如土壤养分循环)提供了重要的理论依据。     

Coupled response of soil carbon and nitrogen pools and enzyme activities to nitrogen and water addition in a semi-arid grassland of Inner Mongolia

Background and aims

Previous studies have demonstrated positive net primary production effects with increased nitrogen (N) and water availability in Inner Mongolian semi-arid grasslands. However, the responses of soil carbon (C) and N concentrations and soil enzyme activities as indicators of impacts of long-term N (urea) and water addition are still unclear. We tested the effect of 7 years of a N and water addition experiment on soil C, N, and specific soil-bound enzymes in a semi-arid grassland of Inner Mongolia.

Methods

We determined concentrations of soil organic carbon (SOC) and soil total nitrogen (TN) in both the 0–10 and 10–20 cm soil layers. Concentrations of labile carbon (LC) and inorganic nitrogen (nitrate and ammonium), and soil pH were measured. Additionally, soil dehydrogenase (DHA), β-glucosidase (BG) and acid and alkaline phosphomonoesterase (PME) enzyme activities were determined in the 0–10 cm soil layer.

Results

SOC concentration in the 0–10 cm soil layer showed no response to N addition or N plus water addition, but increased with water addition alone by 0.3–15.7 %. N addition significantly increased nitrate by 46.0–138.4 % and ammonium by 19.0–73.3 % in the 0–10 cm soil layer, whereas water addition did not affect them. The activities of DHA and alkaline PME enzymes, as well as soil pH, in the 0–10 cm layer decreased with N addition, however water addition alone caused these enzyme activities to increase. Unlike the surface soil (0–10 cm), the lower soil layer (10–20 cm), was responsive to N and water addition in that SOC and TN concentrations decreased with N addition and increased with water addition.

Conclusions

The accumulation of SOC and TN in N and water addition plots may be caused by the input of plant biomass exceeding SOC decomposition. Decrease in microbial activity, derived from decreased DHA and alkaline PME activities might result from suppression effects of lower pH and decreased microbial N supply. Water availability is proved to be more important than N availability for soil C and N accumulation in this semi-arid grassland.     

Soil carbon pool and fertility under natural evergreen broadleaved forest and its artificial regeneration forests in southern Sichuan Province, China

The measurement of total soil organic matter (SOM) is not sensitive enough to detect short and medium term changes, and thus meaningful fractions of SOM should be measured. The carbon management index (CMI) was shown to be a useful technique for describing soil fertility. Soil samples were collected from natural evergreen broadleaved forest and its artificial regeneration forests of Sassafras tzumu, Cryptomeria fortunei and Metasequoia glyptostroboides in southern Sichuan Province, China, to determine soil carbon fractions, available nutrients, enzyme activity and CMI. Regression analysis was used to determine the relationship between soil carbon fractions, CMI and fertility. The results showed that the contents of soil organic carbon, water-soluble carbon, microbial biomass carbon, labile carbon, non-labile carbon, hydrolysis-N, available-P and available-K, the activity of invertase, phosphatase and catalase, and CMI were ranked with different seasons and followed the order: natural evergreen broadleaved forest > Sassafras tzumu plantation > Metasequoia glyptostroboides plantation > Cryptomeria fortunei plantation. The soil carbon fractions and CMI were significantly positively (P < 0.05) correlated with available nutrients and enzyme activity. The results indicate that soil carbon fractions and CMI could be used to evaluate the soil fertility for natural evergreen broadleaved forest and its artificial regeneration forests.     

The effects of long-term fertilization on the accumulation of organic carbon in the deep soil profile of an oasis farmland

Background and aims

Deeper soils represent a poorly understood, but potentially important, sink for carbon sequestration. The objective of this study was to determine the effects of long-term fertilization on soil organic carbon (SOC), its labile fractions and aggregate-associated carbon throughout a 0–3 m soil profile.

Methods

The investigation was conducted in a field experiment started in 1990 in an oasis farmland cropped with winter wheat. The following treatments were compared with the desert from which the oasis was created: CK (no fertilizer), NPK, N2P2K, NPKR, and N2P2R2 (“2” for double fertilizer and “R” for straw residue)

Results

SOC contents increased by 14–56 % in the topsoil (0–0.2 m), but decreased by 15–22 % in the subsoil (0.2–0.6 m) under all fertilizer treatments. In the deep layer (0.6–3 m) there were significant differences between the treatments: SOC decreased by 5–9 % in treatments without straw, but increased by 4–9 % in treatments with straw. Labile fractions (particulate organic carbon and light fraction organic carbon) also showed similar trends. Both the fertilizer and CK treatments led to an increase in the amount of macro-aggregates (>0.25 mm), especially small macro-aggregates (0.25–2 mm), throughout the soil profile. SOC content was highest in the macro-aggregates, intermediate in the silt + clay fraction (<0.053 mm), and lowest in the micro-aggregates (0.25–0.053 mm). However, 44–87 % of total SOC was stored in the silt + clay fraction, especially in the deep layer (at least 80 %).

Conclusions

After 20 years of fertilizer applications, difference in SOC mainly occurred in the deep layer, and preservation of SOC in the silt + clay fraction appeared to be a prerequisite for soil-carbon sequestration. Applying inorganic fertilizer alone decreased SOC content in the silt + clay fraction in the deep layer, while the combined applications with straw resulted in higher SOC content in the silt + clay fraction in that layer, which turned out to be the main mechanism for increasing SOC content. Our study indicated that applying straw with inorganic fertilizer is the best practice for carbon sequestration, which occurred mainly in the deep soil layer.     

Causes of variation in mineral soil C content and turnover in differently managed beech dominated forests

Background and aims

Forest soils are important carbon stores and considered as net CO₂ sinks over decadal to centennial time scales. Intensive forest management is thought to reduce the carbon sequestration potential of forest soils. Here we study the effects of decades of forest management (as unmanaged forest, forest under selection cutting, forest under age class management) on the turnover of mineral associated soil organic matter (MOM) in German beech (Fagus sylvatica L.) dominated forests.

Methods

Radiocarbon contents were determined by accelerator mass spectrometry (AMS) in 79 Ah horizon MOM fractions of Cambisols (n?=?13), Luvisols (n?=?51) and Stagnosols (n?=?15). Mean residence times (MRTs) for soil organic carbon (SOC) were estimated with a 2-pool model using the litter input derived from a forest inventory.

Results

MOM fractions from Ah horizons contained 64?±?8.8 % of the bulk SOC. The radiocarbon content of MOM fractions in Ah horizons, expressed as Δ¹⁴C, ranged between ?2.8?‰ and 114?‰ for the three soil groups. Almost all samples contained a detectable proportion of ‘bomb’ carbon fixed from the atmosphere since 1963. Under the assumption that depending on the soil texture between 19 % and 24 % of the SOC from the labile pool is transferred to the stable SOC pool, the corresponding MRTs ranged between 72 and 723 years, with a median of 164 years.

Conclusions

Our results indicate that the MOM fraction of Ah horizons from beech forests contained a high proportion of young carbon, but we did not find a significant decadal effect of forest management on the radiocarbon signature and related turnover times. Instead, both variables were controlled by clay contents and associated SOC concentrations (p?<?0.01). This underlines the importance of pedogenic properties for SOC turnover in the MOM fraction.     

Converting paddy fields to Lei bamboo (Phyllostachys praecox) stands affected soil nutrient concentrations, labile organic carbon pools, and organic carbon chemical compositions

Background and aims

Land-use change often markedly alters soil carbon (C) and nitrogen (N) pool sizes with implications for climate change and soil sustainability. The objective of this research was to study the effect of converting paddy fields to Lei bamboo (Phyllostachys praecox) stands on soil C and N and other nutrient pools as well as the chemical structure of soil organic C (SOC) in the soil profile.

Methods

Soils (Anthrosols) from four adjacent paddy field–bamboo forest pairs with a known land-use history were sampled from Lin’an County, Zhejiang Province. Soil water soluble organic C (WSOC), hot water soluble organic C (HWSOC), microbial biomass C (MBC), readily oxidizable C (ROC), water soluble organic N (WSON), and other soil chemical and physical properties were determined. Soil organic C functional group compositions were determined by ¹³C-nuclear magnetic resonance (NMR).

Results

Concentrations of soil available P, available K, and different N forms increased (P?<?0.05) by the land-use conversion. Higher concentrations of SOC and total N (TN) were observed in the subsoil (20–40 and 40–60 cm soil layers) but not in the topsoil (0–20 cm layer) in the bamboo stands than in the paddy fields. The storage of SOC and TN in the entire soil profile (0–60 cm) increased by 56.7 and 70.7 %, respectively, after the land-use change. The increases in the SOC stock of the three soil layers were 11.0, 14.3, and 9.5 Mg C ha^?1, respectively. The conversion decreased WSOC concentrations in the subsoil but increased the ROC concentration in the topsoil. Solid-state NMR spectroscopy of soil samples showed that the conversion increased (P?<?0.05) the O-alkyl C content while decreased the aromatic C content, alkyl C to O-alkyl C ratio (A/O-A), and aromaticity of SOC.

Conclusions

Conversion of paddy fields to bamboo stands increased soil nutrient availability, and SOC and TN stocks. Effects of land-use change on C pools and C chemistry of SOC varied among different soil layers in the profile. The impact of the land-use conversion on soil organic C pools was not restricted to the topsoil, but changes in the subsoil were equally large and should be accounted for.     

Lignin properties in topsoils of a beech/oak forest after 8 years of manipulated litter fall: relevance of altered input and oxidation of lignin

Background and aims

We studied the response of lignin oxidation in soils of a beech/oak forest to changes in litter fall. Additionally we considered possible factors in lignin oxidation, including altered (i) input of fresh organic matter and (ii) fungi-to-bacteria ratios.

Methods

The field-based experiment included (i) doubling and (ii) exclusion of litter fall and (iii) controls with ambient litter fall. Soil (0–20 cm depth) was sampled after 8 years. We analyzed (i) lignin using the CuO oxidation method, (ii) stocks of free and mineral-bound organic carbon (OC), (iii) the response of soil organic matter (SOM) decomposition to addition of labile organic compounds in laboratory incubations, and (iv) ratios of fungal- vs. bacterial-derived amino sugars (F/B ratios).

Results

Litter exclusion increased stocks of free-light fraction OC, F/B ratios, the ability of the microbial community to use labile compounds for SOM decomposition, as well as acid-to-aldehyde ratios of vanillyl-type lignin phenols in A horizons. Litter addition had no such effects. We assume that litter exclusion caused enhanced transport of organic debris from lower forest floor horizons with rainwater into the A horizon. Enhanced input of organic debris might have increased (i) the availability of labile compounds and (ii) F/B ratios. Consequently, lignin oxidation increased.

Conclusions

Enhanced input of organic debris from forest floors can increase lignin oxidation in mineral topsoils of the studied forest. The expected gradual changes in litter fall due to climate change likely will cause no such effects.     

Incorporation of Plant Residues into Soil Organic Matter Fractions With Grassland Management Practices in the North American Midwest

Disturbed grassland soils are often cited as having the potential to store large amounts of carbon (C). Fertilization of grasslands can promote soil C storage, but little is known about the generation of recalcitrant pools of soil organic matter (SOM) with management treatments, which is critical for long-term soil C storage. We used a combination of soil incubations, size fractionation and acid hydrolysis of SOM, [C], [N], and stable isotopic analyses, and biomass quality indices to examine how fertilization and haying can impact SOM dynamics in Kansan grassland soils. Fertilized soils possessed 113% of the C possessed by soils subjected to other treatments, an increase predominantly harbored in the largest size fraction (212–2,000 μm). This fraction is frequently associated with more labile material. Haying and fertilization/haying, treatments that more accurately mimic true management techniques, did not induce any increase in soil C. The difference in ¹⁵N-enrichment between size fractions was consistent with a decoupling of SOM processing between pools with fertilization, congruent with gains of SOM in the largest size fraction promoted by fertilization not moving readily into smaller fractions that frequently harbor more recalcitrant material. Litterfall and root biomass C inputs increased 104% with fertilization over control plots, and this material possessed lower C:N ratios. Models of incubation mineralization kinetics indicate that fertilized soils have larger pools of labile organic C. Model estimates of turnover rates of the labile and recalcitrant C pools did not differ between treatments (65.5 ± 7.2 and 2.9 ± 0.3 μg C d⁻¹, respectively). Although fertilization may promote greater organic inputs into these soils, much of that material is transformed into relatively labile forms of soil C; these data highlight the challenges of managing grasslands for long-term soil C sequestration.     

Soil carbon pool and fertility under natural evergreen broadleaved forest and its artificial regeneration forests in southern Sichuan Province, China

The measurement of total soil organic matter (SOM) is not sensitive enough to detect short and medium term changes, and thus meaningful fractions of SOM should be measured. The carbon management index (CMI) was shown to be a useful technique for describing soil fertility. Soil samples were collected from natural evergreen broadleaved forest and its artificial regeneration forests of Sassafras tzumu, Cryptomeria fortunei and Metasequoia glyptostroboides in southern Sichuan Province, China, to determine soil carbon fractions, available nutrients, enzyme activity and CMI. Regression analysis was used to determine the relationship between soil carbon fractions, CMI and fertility. The results showed that the contents of soil organic carbon, water-soluble carbon, microbial biomass carbon, labile carbon, non-labile carbon, hydrolysis-N, available-P and available-K, the activity of invertase, phosphatase and catalase, and CMI were ranked with different seasons and followed the order: natural evergreen broadleaved forest > Sassafras tzumu plantation > Metasequoia glyptostroboides plantation > Cryptomeria fortunei plantation. The soil carbon fractions and CMI were significantly positively (P < 0.05) correlated with available nutrients and enzyme activity. The results indicate that soil carbon fractions and CMI could be used to evaluate the soil fertility for natural evergreen broadleaved forest and its artificial regeneration forests.     

The Effects of Manure and Nitrogen Fertilizer Applications on Soil Organic Carbon and Nitrogen in a High-Input Cropping System

With the goal of improving N fertilizer management to maximize soil organic carbon (SOC) storage and minimize N losses in high-intensity cropping system, a 6-years greenhouse vegetable experiment was conducted from 2004 to 2010 in Shouguang, northern China. Treatment tested the effects of organic manure and N fertilizer on SOC, total N (TN) pool and annual apparent N losses. The results demonstrated that SOC and TN concentrations in the 0-10cm soil layer decreased significantly without organic manure and mineral N applications, primarily because of the decomposition of stable C. Increasing C inputs through wheat straw and chicken manure incorporation couldn''t increase SOC pools over the 4 year duration of the experiment. In contrast to the organic manure treatment, the SOC and TN pools were not increased with the combination of organic manure and N fertilizer. However, the soil labile carbon fractions increased significantly when both chicken manure and N fertilizer were applied together. Additionally, lower optimized N fertilizer inputs did not decrease SOC and TN accumulation compared with conventional N applications. Despite the annual apparent N losses for the optimized N treatment were significantly lower than that for the conventional N treatment, the unchanged SOC over the past 6 years might limit N storage in the soil and more surplus N were lost to the environment. Consequently, optimized N fertilizer inputs according to root-zone N management did not influence the accumulation of SOC and TN in soil; but beneficial in reducing apparent N losses. N fertilizer management in a greenhouse cropping system should not only identify how to reduce N fertilizer input but should also be more attentive to improving soil fertility with better management of organic manure.     

Roles of shoots and roots in cadmium uptake and distribution in tubers of potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis> L)

Aims

We investigated whether density fractionation can be used to determine the distribution of organic phosphorus (OP) between free and mineral-associated soil organic matter (SOM).

Methods

We performed density fractionations using sodium polytungstate solution (specific gravity 1.6 g cm^?3) on 20 soils from UK semi-natural and pasture ecosystems, to obtain a light fraction (LF) and a heavy fraction (HF) for each soil. The fractions were quantified by weight, and analysed for organic carbon (OC), total N (TN), total P (TP), inorganic P (IP), and OP (by difference).

Results

Good recoveries of soil mass (96%), OC and TN (both ~ 90%) were obtained, but recovery of OP only averaged 56%. The average P:C ratio of HF SOM exceeded that of LF SOM by a factor of six, greater than the factor of two obtained for TN:OC. For the soils studied, the elements of SOM were predominantly in the HF, with averages of 75% for C, 82% for N, and 90% for P.

Conclusions

The incomplete recovery of OP demands further work. Nonetheless, the results show that HF SOM is much richer in P than LF SOM.

     

长期施肥对黄土旱塬农田土壤有机磷组分及小麦产量的影响

研究长期施肥对黄土旱塬农田土壤有机磷组分及小麦产量的影响,可为提高磷素转化利用率及合理利用肥料提供理论支持。本研究依托长武旱塬农田生态系统长期(1984—2016年)定位试验站,选取不施肥(CK)、单施氮肥(N)、单施磷肥(P)、施氮磷肥(NP)、单施有机肥(M)、氮肥配施有机肥(MN)、磷肥配施有机肥(MP)、氮磷肥配施有机肥(MNP)8个处理,研究其对土壤有机磷组分、小麦产量和土壤性质的影响。结果表明: 长期施肥后土壤有机磷含量为244.7～429.1 mg·kg^-1,除N处理外,其余各处理有机磷含量比CK均显著增加了15.4%～47.9%。长期施用磷肥改变了黄土旱塬农田表层土壤(0～20 cm)各有机磷组分含量,MP、MNP处理显著提高了活性有机磷及中活性有机磷含量;N、P和NP处理显著降低了中稳性有机磷含量;N、P、NP、MN、MP、MNP处理均显著提高了高稳性有机磷含量。各处理土壤有机磷组分与总有机磷含量比值为:中活性有机磷>高稳性有机磷>活性有机磷>中稳性有机磷。长期施肥后,与CK相比,氮、磷肥配施,尤其是与有机肥配施,显著增加了小麦生物产量和籽粒产量。土壤指标中,有机质、速效磷和无机磷与小麦产量呈显著正相关。MP、M处理可以显著提高黄土旱塬黑垆土中的速效磷、总磷、总无机磷、活性有机磷和中活性有机磷含量,表明有机肥与磷肥配施可以提高该地区更容易被作物吸收的磷组分。总之,氮磷肥配施并配施有机肥可以提高该地区磷供给,对小麦增产有促进作用,对提高黄土旱塬地区土壤质量有重要意义。     

Interactions between leaf litter and soil organic matter on carbon and nitrogen mineralization in six forest litter-soil systems

Background and aims

Leaf litter decomposes on the surface of soil in natural systems and element transfers between litter and soil are commonly found. However, how litter and soil organic matter (SOM) interact to influence decomposition rate and nitrogen (N) release remains unclear.

Methods

Leaf litter and mineral soil of top 0–5 cm from six forests were incubated separately, or together with litter on soil surface at 25 °C for 346 days. Litter N remaining and soil respiration rate were repeatedly measured during incubation. Litter carbon (C) and mass losses and mineral N concentrations in litter and soil were measured at the end of incubation.

Results

Net N transfer from soil to litter was found in all litters when incubated with soil. Litter incubated with soil lost more C than litter incubated alone after 346 days. For litters with initial C: N ratios lower than 52, net N_min after 346 days was 100 % higher when incubated with soil than when incubated alone. Litter net N_min rate was negatively related to initial C: N ratio when incubated with soil but not when incubated alone. Soil respiration rate and net N_min rate did not differ between soil incubated with litter and soil incubated alone.

Conclusions

We conclude that soils may enhance litter decomposition rate by net N transfer from soil to litter. Our results together with studies on litter mixture decomposition suggest that net N transfer between decomposing organic matter with different N status may be common and may significantly influence decomposition and N release. The low net N_min rate during litter decomposition along with the small size of litter N pool compared to soil N pool suggest that SOM rather than decomposing litter is the major contributor to plant mineral N supply.     

Crop Management for Soil Carbon Sequestration

Reducing emissions of greenhouse gases (GHG) from agriculture is related to increasing and protecting soil organic matter (SOM) concentration. Agricultural soils can be a significant sink for atmospheric carbon (C) through increase of the SOM concentration. The natural ecosystems such as forests or prairies, where C gains are in equilibrium with losses, lose a large fraction of the antecedent C pool upon conversion to agricultural ecosystems. Adoption of recommended management practices (RMPs) can enhance the soil organic carbon (SOC) pool to fill the large C sink capacity on the world's agricultural soils. This article collates, reviews, and synthesizes the available information on SOC sequestration by RMPs, with specific references to crop rotations and tillage practices, cover crops, ley farming and agroforestry, use of manure and biosolids, N fertilization, and precision farming and irrigation. There is a strong interaction among RMPs with regards to their effect on SOC concentration and soil quality. The new equilibrium SOC level may be achieved over 25 to 50 years. While RMPs are being adapted in developed economies, there is an urgent need to encourage their adoption in developing countries. In addition to enhancing SOC concentration, adoption of RMPs also increases agronomic yield. Thus, key to enhancing soil quality and achieving food security lies in managing agricultural ecosystems using ecological principles which lead to enhancement of SOC pool and sustainable management of soil and water resources.     

Long-Term Effect of Manure and Fertilizer on Soil Organic Carbon Pools in Dryland Farming in Northwest China

An understanding of the dynamics of soil organic carbon (SOC) as affected by farming practices is imperative for maintaining soil productivity and mitigating global warming. The objectives of this study were to investigate the effects of long-term fertilization on SOC and SOC fractions for the whole soil profile (0–100 cm) in northwest China. The study was initiated in 1979 in Gansu, China and included six treatments: unfertilized control (CK), nitrogen fertilizer (N), nitrogen and phosphorus (P) fertilizers (NP), straw plus N and P fertilizers (NP+S), farmyard manure (FYM), and farmyard manure plus N and P fertilizers (NP+FYM). Results showed that SOC concentration in the 0–20 cm soil layer increased with time except in the CK and N treatments. Long-term fertilization significantly influenced SOC concentrations and storage to 60 cm depth. Below 60 cm, SOC concentrations and storages were statistically not significant between all treatments. The concentration of SOC at different depths in 0–60 cm soil profile was higher under NP+FYM follow by under NP+S, compared to under CK. The SOC storage in 0–60 cm in NP+FYM, NP+S, FYM and NP treatments were increased by 41.3%, 32.9%, 28.1% and 17.9%, respectively, as compared to the CK treatment. Organic manure plus inorganic fertilizer application also increased labile soil organic carbon pools in 0–60 cm depth. The average concentration of particulate organic carbon (POC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) in organic manure plus inorganic fertilizer treatments (NP+S and NP+FYM) in 0–60 cm depth were increased by 64.9–91.9%, 42.5–56.9%, and 74.7–99.4%, respectively, over the CK treatment. The POC, MBC and DOC concentrations increased linearly with increasing SOC content. These results indicate that long-term additions of organic manure have the most beneficial effects in building carbon pools among the investigated types of fertilization.     

Ecosystem carbon storage and soil organic carbon stability in pure and mixed stands of Cunninghamia lanceolata and Michelia macclurei

Background and aims

Across the world, about 264 million ha forest plantations are monospecific. This practice has been found to cause site productivity and soil fertility decline in the regions where forests have been harvested several times. To mitigate these problems, mixed-species plantations, especially with broadleaved and coniferous species, are preferred. Understanding the effects of introducing broadleaved tree in monospecific coniferous plantation on ecosystem carbon (C) storage and soil organic C (SOC) stability is critical to improve our understanding of forest C sequestration and C cycle.

Methods

Plots were established in subtropical plantations with a randomized block design to examine the influence of introducing Michelia macclurei trees into pure Cunninghamia lanceolata plantation on biomass C storage, SOC storage of total, labile, and recalcitrant fractions (0–40 cm depth), and SOC stability.

Results

Introducing M. macclurei trees increased biomass C by 17.9 % and 14.2 % compared with monospecific C. lanceolata and M. macclurei plantations, respectively. Storage of different SOC fractions was not significantly different between monospecific C. lanceolata and mixed plantations. SOC stability in bulk soils was not affected, although it differed in 10–20 cm and 20–40 cm soil depth among three plantations.

Conclusions

Mixed species plantations can increase C sequestration, and in the subtropical forest ecosystem examined this was mainly attributed to an increase in biomass C.     

Nutrient availability and pH jointly constrain microbial extracellular enzyme activities in nutrient-poor tundra soils

Background and aims

Tundra soils, which usually contain low concentrations of soil nutrients and have a low pH, store a large proportion of the global soil carbon (C) pool. The importance of soil nitrogen (N) availability for microbial activity in the tundra has received a great deal of attention; however, although soil pH is known to exert a considerable impact on microbial activities across ecosystems, the importance of soil pH in the tundra has not been experimentally investigated.

Methods

We tested a hypothesis that low nutrient availability and pH may limit microbial biomass and microbial capacity for organic matter degradation in acidic tundra heaths by analyzing potential extracellular enzyme activities and microbial biomass after 6 years of factorial treatments of fertilization and liming.

Results

Increasing nutrients enhanced the potential activity of β-glucosidase (synthesized for cellulose degradation). Increasing soil pH, in contrast, reduced the potential activity of β-glucosidase. The soil phospholipid fatty acid concentrations (PLFAs; indicative of the amount of microbial biomass) increased in response to fertilization but were not influenced by liming.

Conclusions

Our results show that soil nutrient availability and pH together control extracellular enzyme activities but with largely differing or even opposing effects. When nutrient limitation was alleviated by fertilization, microbial biomass and enzymatic capacity for cellulose decomposition increased, which likely facilitates greater decomposition of soil organic matter. Increased soil pH, in contrast, reduced enzymatic capacity for cellulose decomposition, which could be related with the bioavailability of organic substrates.     

Variation of soil carbon pools in Pinus sylvestris plantations of different ages in north China

Plantations play an important role in absorbing atmospheric CO₂ and plantation soil can serve as an important carbon (C) sink. However, the stocks and dynamics of soil C in differently aged plantation forests in north China remain uncertain. In this study, we measured soil inorganic carbon (SIC), soil organic carbon (SOC) and total nitrogen content (STN), the light (LF) and heavy fractions (HF) of soil organic matter (SOM) to a depth of 1 m in 3 different ages (10-, 30-, 40-year-old) of Pinus sylvestris var. mongolica (Mongolia pine) plantations in 2011 and 2012. Soil pH, texture and moisture were also measured to explore the causes of SOC dynamics for different stand ages. Our results showed that no significant difference in SIC content was observed at different soil depths. As forest age increases, SIC content as well as the C and N content in SOM, LF and HF initially rose and then decreased, while the LF in SOC initially decreased and then increased. Although the C:N ratio of SOC and HF did not significantly change, the C:N ratio of LF increased with depth. SOC dynamics at different stand ages were significantly correlated with soil moisture and clay content. Soil pH and moisture explained 58.63% of the overall variation of SOC at different depths. Moreover, the SOC increased during the early stage of afforestation, mostly because of the increase in recalcitrant C; however, the decrease of SOC with increasing stand age was also mainly affected by C loss in the recalcitrant C pool.     

Long term fertilization effects on soil organic carbon pools in a sandy loam soil of the Indian sub-Himalayas

An understanding of the dynamics of soil organic carbon (SOC) as affected by farming practices is imperative for maintaining soil productivity and mitigating global warming. Results of a long-term (32 years) experiment in the Indian Himalayas under rainfed soybean (Glycine max L.)- wheat (Triticum aestivum L.) rotation was analyzed to determine the effects of mineral fertilizer and farmyard manure (FYM) application at 10 Mg?ha^-1 on SOC stocks and depth distribution of the labile and recalcitrant pools of SOC. Results indicate all treatments increased SOC contents over the control. The annual application of NPK significantly (P?<?0.05) enhanced total SOC, oxidizable soil organic C and its fractions over the control plots. The increase in these SOC fractions was greater with the NPK + FYM treatment. Nearly 16% (mean of all treatments) of the estimated added C was stabilized into SOC both in the labile and recalcitrant pools, preferentially in the 0?C30 cm soil layer. However, the labile:recalcitrant SOC ratios of applied C stabilized was largest in the 15?C30 cm soil layer. About 62% of total SOC was present in the labile pool. Plots under the N + FYM and NPK + FYM treatments contained a larger proportion of total SOC in the recalcitrant pool than the plots with mineral or no fertilizer, indicating that FYM application promoted SOC stabilization.     

Fertilization enhancing carbon sequestration as carbonate in arid cropland: assessments of long-term experiments in northern China

Aims

Soil inorganic carbon (SIC), primarily calcium carbonate, is a major reservoir of carbon in arid lands. This study was designed to test the hypothesis that carbonate might be enhanced in arid cropland, in association with soil fertility improvement via organic amendments.

Methods

We obtained two sets (65 each) of archived soil samples collected in the early and late 2000’s from three long-term experiment sites under wheat-corn cropping with various fertilization treatments in northern China. Soil organic (SOC), SIC and their Stable ¹³C compositions were determined over the range 0–100 cm.

Results

All sites showed an overall increase of SIC content in soil profiles over time. Particularly, fertilizations led to large SIC accumulation with a range of 101–202 g C m^?2 y^?1 in the 0–100 cm. Accumulation of pedogenic carbonate under fertilization varied from 60 to 179 g C m^?2 y^?1 in the 0–100 cm. Organic amendments significantly enhanced carbonate accumulation, in particular in the subsoil.

Conclusions

More carbon was sequestrated in the form of carbonate than as SOC in the arid cropland in northern China. Increasing SOC stock through long-term straw incorporation and manure application in the arid and semi-arid regions also enhanced carbonate accumulation in soil profiles.