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Climate warming and anthropogenic nitrogen (N) loads are two major global change components interactively affecting carbon cycling. However, the effects of N forms and amounts on temperature sensitivity (Q10) of soil organic matter (SOM) mineralization remain incomplete. With this goal, soil was sampled after 23 years of mineral and (or) organic N fertilization, and then incubated for one year at 10, 20, and 30°C. For the first time, we compared four approaches (Equal time, Equal C, 1‐C pool, and 2‐C pool model) to evaluate the Q10 of SOM mineralization. All approaches showed that the Q10 decreased by more than one third with N fertilization compared to unfertilized control at low temperatures. The '1‐C pool model' was not adequate for Q10 estimation with various C availability. The Q10 estimated by '2‐C pool model' was strongly depended on incubation duration. The 'Equal C' approach was more powerful for separating SOM pools and it revealed the decreased Q10 of the recalcitrant pool at high N rates. The impact of N fertilization on Q10 was more evident at high N than at low N. Notably, the Q10 decreased more by mineral N compared to organic fertilizers (~60% vs. ~40% decreased in Q10) at 10–20oC. The added benefit of N fertilization in protecting SOM under climate warming was demonstrated by decreased Q10. Such one‐third reduction of temperature sensitivity by N fertilization is large enough to be considered in predictions of global SOM stocks under warming and anthropogenic N loads. [ABSTRACT FROM AUTHOR]

Aims: Understanding the effects of long-term crop management on soil organic matter (SOM) is necessary to improve the soil quality and sustainability of agroecosystems. Method: The present 7-year long-term field experiment was conducted to evaluate the effect of integrated management systems and N fertilization on SOM fractions and carbon management index (CMI). Two integrated soil-crop system management (ISSM-1 and ISSM-2, combined with improved cultivation pattern, water management and no-tillage) were compared with a traditional farming system at three nitrogen (N) fertilization rates (0, 150 and 225 kg N ha). Results: Management systems had greater effects on SOM and its fractions than did N fertilization. Compared with traditional farming practice, the integrated management systems increased soil organic carbon (SOC) by 13 % and total nitrogen (TN) by 10 % (averaged over N levels) after 7 years. Integrated management systems were more effective in increasing labile SOM fractions and CMI as compared to traditional farming practice. SOC, TN and dissolved organic matter in nitrogen increased with N fertilization rates. Nonetheless, N addition decreased other labile fractions: particulate organic matter, dissolved organic matter in carbon, microbial biomass nitrogen and potassium permanganate-oxidizable carbon. Conclusions: We conclude that integrated management systems increased total SOM, labile fractions and CMI, effectively improved soil quality in rice-rapeseed rotations. Appropriate N fertilization (N150) resulted in higher SOC and TN. Though N application increased dissolved organic matter in nitrogen, it was prone to decrease most of the other labile SOM fractions, especially under higher N rate (N250), implying the decline of SOM quality. [ABSTRACT FROM AUTHOR]

Abstract: Labile soil organic matter (SOM) fractions are especially important because they are more vulnerable to disturbance and play a crucial role for nutrient and carbon cycling. Although water conservation has become increasingly important in rice–wheat rotation, the effects of non-flooded mulching cultivation on labile SOM fractions remain unknown. Based on a long-term field experiment (10 years), we analyzed the impact of non-flooded mulching cultivation and cropping season on labile SOM fractions in a rice–wheat rotation in Chengdu Plain, southwest China. Compared with traditional flooding (TF), the plastic film mulching (PM) and wheat straw mulching (SM) treatments increased dissolved organic carbon (DOC) (42% after rice season and 41% after wheat season), but decreased microbial biomass carbon (MBC) (19% after rice season and 28% after wheat season) and nitrogen (MBN) (17% after rice season and 24% after wheat season) in the 0–5 cm depth. SM increased particulate organic carbon (POC) and KMnO4-oxidizable C (KMnO4-C) contents after both the rice and wheat seasons. Microbial biomass and DOC concentrations were higher for all three cultivations after the rice season than after the wheat season. In contrast, POC contents under PM and SM were higher after the wheat season than after the rice season. In general, results in this study indicate that non-flooded mulching and cropping season significantly influenced labile SOM fractions. The DOC fraction was the most sensitive fraction affected by non-flooded mulching, while POC and PON fractions respond fast within the two cropping seasons. [Copyright &y& Elsevier]

Abstract: The conversion from cereal fields to vegetable production in the last three decades represents a significant shift in land use in China. Here, we studied the effects of conversion form cereal fields to vegetable production in north China on soil organic carbon (SOC) and total nitrogen (TN) in both bulk soil and soil aggregates. We used two approaches: (1) measurements of paired soil samples from wheat (Triticum aestivum L.) – maize (Zea mays L.) fields and adjacent greenhouses vegetable fields in three vegetable production areas representing various management intensities in terms of C and N inputs and frequency of tillage; (2) fractionating soil to distinguish intra-aggregate particulate organic matter (iPOM) and organo-mineral complexes (silt+clay). Our results indicated that converting cereal fields to greenhouse vegetable production with intermediate and high management intensity led to increases in SOC and TN and decreases in C:N ratios in the top soil. The accumulation rates of C and N in the surface soil (0–30cm) were estimated to be 1.37MgCha−1 yr−1 and 0.21MgNha−1 yr−1 over an average period of 8 years after cereal fields to greenhouse vegetable production conversion. At the soil aggregate level, only the coarse (>250μm) and fine (53–250μm) iPOM fraction contributed to the increases in soil C (e.g., 49% and 51% of total C increases, respectively), while the coarse and fine iPOM, and silt+clay fraction accounted for 22%, 30% and 48%, respectively, of total N increases. This illustrates how the addition of readily available C (manure) and N (manure and inorganic N) leads to a temporary stabilization of C in relatively labile SOM fractions, but to a preferential stabilization of N in organo-mineral SOM fractions. In conclusion, the conversion to highly intensive vegetable systems in China leads to marked differences in C and N stabilization dynamics. [Copyright &y& Elsevier]

A comprehensive synthesis of 32 long-term experiments in China is performed by comparing yields and SOM contents and their time by nutrient management responses (TNMR) to determine whether and to what extent combined application of organics amendments and fertilizers (organics + fertilizers) can increase productivity and soil organic matter (SOM) content and enhance the sustainability of diverse cropping systems. TNMR is defined as percentage changes over time in yields and SOM contents in external nutrient input treatments relative to initial and control plots and represents the net effects of nutrient management strategies on yields and SOM contents. Organics + fertilizers significantly increased yields over three crops (wheat, maize and rice) on average by 29% relative to sole organics and 8% to fertilizer only. Higher TNMR values in yields were found with organics + fertilizers in the sequence wheat (53%) > maize (40%) > rice (8%). Significant and positive time trends in TNMR of organics + fertilizers in wheat and maize yield may imply enhanced sustainability of cropping systems. Addition of organics (alone or combined with fertilizers) increased SOM contents and their TNMR over fertilizers applied alone. However, benefits varied between organics and organics + fertilizers by land use type. The latter led to higher SOM contents than (for dry land) and similar to the former (for dry-flooded and flooded systems). 2.3 and 1.4 times higher TNMR of organics + fertilizers in SOM content in dry land than dry-flooded and flooded systems implies that dry land was more responsive in SOM content increase upon adoption of organics + fertilizers than the latter. Overall, despite variation in the quantitative magnitude in crop or land use type, compared with application of organics or fertilizers only, a combination of both represents the most effective way to produce more food, build up SOM and enhance sustainability, especially in dry land cropping systems. [ABSTRACT FROM AUTHOR]

Abstract: Soil organic matter (SOM) and enzymes are essential for nutrient cycling, and are considered as important indicators of soil quality. The effects of organic and mineral fertilization on soil organic carbon (SOC), total nitrogen (TN) and enzyme activities in bulk soil and particle-size fractions were investigated under a winter wheat/maize cropping system in the North China Plain. The experiment established in 1993 includes three treatments: (1) unfertilized control (CK); (2) mineral fertilizers (MF); and (3) farmyard manure (FYM). Application of FYM significantly increased SOC and TN contents and activities of six enzymes: invertase, β-glucosidase, urease, acid and alkaline phosphatases and dehydrogenase in bulk soil and in all particle-size fractions as compared to those in MF and CK. Highest contents of SOC and TN were found in coarse sand and lowest in the silt fraction. The C/N ratios decreased with decreasing particle-size fractions. β-Glucosidase and acid phosphatase activities predominated in coarse sand fraction, reflecting high substrate availability. The urease activity was highest in clay-size fractions, depending on mineral sorption processes. The SOM and enzyme activities in the coarse sand were the most sensitive to fertilization. The smallest response of SOM in the clay fraction to fertilization confirmed that SOM on clay is the most stable C pool. The 15-year fertilization experiment clearly showed that FYM represented the best management practice for improving soil quality and microbial activity. [Copyright &y& Elsevier]

Abstract: The Loess Plateau in northwest China is one of the most eroded landscapes in the world, and it is urgent that alternative practices be evaluated to control soil erosion. Our objective was to determine how three different tillage practices for monoculture of winter wheat (Triticum aestivum L.) affected soil organic carbon (SOC) and N content after 11 years. Conventional tillage with residue removal (CT), shallow tillage with residue cover (ST), and no-tillage with residue cover (NT) were investigated. Carbon and N in various aggregate-size classes and various labile organic C fractions in the 0–15- and 15–30-cm soil layers were evaluated. The ST and NT treatments had 14.2 and 13.7% higher SOC stocks and 14.1 and 3.7% higher total N (Nt) stocks than CT in the upper 15cm, respectively. Labile C fractions: particulate organic C (POC), permanganate oxidizable C (KMnO4-C), hot-water extractable C (HWC), microbial biomass C (MBC) and dissolved organic C (DOC) were all significantly higher in NT and ST than in CT in the upper 15cm. KMnO4-C, POC and HWC were the most sensitive fractions to tillage changes. The portion of 0.25–2mm aggregates, mean weight diameter (MWD) and geometric mean diameter (GMD) of aggregates from ST and NT treatments were larger than from CT at both 0–15- and 15–30-cm soil depths. The ST and NT treatments had significantly higher SOC and Nt in the 0.25–2mm fraction at both depths and significantly higher Nt content in the upper 15cm. Positive significant correlations were observed between SOC, labile organic C fractions, MWD, GMD, and macroaggregate (0.25–2mm) C within the upper 15cm. We conclude that both variants of conservation tillage (NT and ST) increase SOC stock in the rainfed farming areas of northern China and are therefore more sustainable practices than those currently being used. [Copyright &y& Elsevier]

• Reduced tillage, residue retention increased C and N versus conventional management. • Enzyme activity increased with conservation management. • Conservation management increased enzyme activities in all particle-size fractions. • Magnitude of increases greater for enzyme activities than C and N stocks. • Enzyme activity sensitive indicator of soil health in response to management change. Soil organic matter (SOM) concentration and enzyme activity are important biochemical indicators of soil health for assessing the sustainability of agricultural management practices. However, little is known about the long-term effects of tillage and crop residue management on SOM and enzyme activities in soil particle-size fractions on the Loess Plateau of Northern China. The objective of this study was to investigate the effects of 11 years of combined tillage and crop residue management treatments on soil organic carbon (SOC), total nitrogen (TN) concentrations and enzyme activities in bulk soil and particle-size fractions from a rainfed wheat (Triticum aestivum L.) monoculture system in this region. We hypothesized that reduced tillage and increased residue retention would increase SOC, TN and enzyme activities in both bulk soil and particle-size fractions, and that enzyme activity would serve as a more sensitive indicator of soil health in response to management. Compared with conventional tillage and residue removal (CTRR), reduced tillage and stubble mulch residue retention (RTSM) increased bulk soil activities of most enzymes (sulfatase +68%, invertase +62%, β-glucosidase +58%, dehydrogenase +46%). These increases were greater than the relative increases in total SOC (34%) and TN (33%) concentrations, supporting our hypothesis of a stronger response in microbial activity to management than total element stocks. The RTSM treatment also increased SOC and TN concentrations, as well as β-glucosidase, acid phosphatase and urease activities in all particle-size fractions (2000-250, 250-53, 53-2 and < 2 μm) compared with the CTRR treatment. Both β-glucosidase and acid phosphatase showed a general decrease from coarse- to fine-sized fractions, and resembled the distribution of SOC and TN concentrations in particle-size fractions. Conversely, urease activity was greater in sand and clay fractions, which was decoupled from SOC and TN distributions. Our results indicate that biological indicators of soil health were more sensitive than C and N stocks to cumulative long-term changes in tillage and residue management. [ABSTRACT FROM AUTHOR]

Global warming and increasing anthropogenic nitrogen (N) loads are two major worldwide issues that arose with industrialization, and both factors permanently impact carbon (C) cycling. For the first time, we compared four independent approaches to evaluate the temperature sensitivity (Q10) of soil organic matter (SOM) mineralization: Equal time, Equal C, 1-C pool model and 2-C pool model. With this goal, soil was sampled from a wheat-maize double cropping system, after 23-year of no N (control), low N (organic and mineral N), high organic N (manuring), or high mineral N fertilization. Four Q10 estimation approaches were compared based on the CO2 released from long-term N fertilized soil at 10, 20 and 30 ºC during one-year incubation. All four approaches clearly showed large Q10 decrease for all SOM pools by N fertilization compared to unfertilized control (Q10 = 0.8-2.3 vs. 2.7-3.7) at low temperature. N fertilization decreased the activation energy (from 85 to 56 kJ mol-1) and led to lower Q10 according to Arrhenius formalism. The lower Q10 under long-term N fertilization resulted also in the decreased C:N ratio, accelerated C turnover and increased microbial C use efficiency. All Q10 estimation approaches captured the decomposition of different SOM pools (from labile to recalcitrant) at various time scales. The '1-C pool model' is not adequate for Q10 estimation because it ignores the response of various C pools to soil warming. The '2-C pool model' fit data very well but the fitted decomposition rates strongly depend on incubation duration. The 'Equal C' approach was more dynamic in separating various SOM to provide a mechanistic understanding of soil C sequestration and cycling under increasing temperature. However, 'Equal C' approach was largely C quality dependent and disregards the incubation duration. The advantages and shortcomings of the four Q10 estimation approaches were intensively discussed. Long-term incubation and continuous measurement of SOM mineralization is always needed to separate various C pools, especially very recalcitrant C. N fertilization lead to added benefit in protecting SOM under warming by decreasing Q10 of SOM. Such reduction of temperature sensitivity caused by N fertilization is large enough to be considered in predictions of the magnitude of global SOM stocks under warming and increasing anthropogenic N loads. [ABSTRACT FROM AUTHOR]