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The effect of grassland conversion to cropland on the incorporation of N into microbial residues remains unclear. We analyzed for amino sugars, as a microbial residue biomarker, and adopted stable-isotope-probing technique to trace the assimilation of 15NH4+-N (added in the form of 15 N-labelled ammonium sulphate) into amino sugars with or without organic C sources (glucose and crop residue) in native grassland and converted cropland soils over a 65-day period. Total 15 N-amino sugars in the cropland were significantly decreased by 16–29% compared to grassland regardless of C source. This suggests that the microbial assimilation of available N into microbial residues was decreased, after conversion, and that stronger responses to substrate inputs occurred in the grassland soils. The addition of available and complex C substrates triggered divergent accumulation patterns of 15 N-amino sugars, indicating that C availability was a major driver for microbial N immobilization. Meanwhile, the conversion of grassland led to suppressed N immobilization activity of both fungi and bacteria. These results suggest that long-term land-use change could considerably affect the N utilization by fungi and bacteria during their anabolic processes, which may exert long-lasting impacts on soil organic N storage.

High altitude alpine grasslands in the Qinghai-Tibetan Plateau (QTP) contain high soil organic C (SOC) stocks that are extremely vulnerable to climate warming. Microbial residues are increasingly recognized as a major source of SOC, however, how climate warming affects this component of SOC in this region remains largely unknown. In this study, we examined the response of microbial residues to a 3-year experimental warming and the degree to which they contributed to SOC storage in two Tibetan ecosystems—alpine steppe (AS) and swamp meadow (SM). The number of microbial residues was indicated by amino sugar analysis. Our results revealed that warming yielded divergent microbial residue accumulation that significantly altered their contribution to SOC storage in the two alpine grasslands. Warming increased microbial residue abundance by approximately 17.6% across 0 to 20 cm depth in SM soils, while causing a significant decline (about 6.2%) in AS soils. The higher microbial residue accumulation in SM could lessen potential positive feedbacks from climate warming, while the decrease in microbial residues in AS may indicate greater loss of microbial-derived C inputs in warmed soils. Moreover, we found that warming selectively increased fungal residues as compared with bacterial despite inconsistent responses to warming in the two grasslands. These changes were accompanied by significant shifts in fungal to bacterial residue C ratios and their contribution to SOC pool, indicating an alteration of SOC composition and stability in alpine grassland ecosystems. These findings demonstrate that a microbial-derived C feedback to climate change is ecosystem-specific that alters the direction and magnitude of the microbial community.

We evaluated how microbial residues and their contributions to soil carbon (C) stocks changed with long-term (50 years) conversion of native grassland to cropland in the profiles (0–15, 15–30, 30–60, 60–90 cm) of chernozem. The conversion of grassland into arable land led to substantial depletion of microbial residues down to 90-cm depth, indicating the potential vulnerability of the microbial-derived C due to land-use change. Moreover, losses of microbial residue C at depths below 30 cm were much higher than that of total soil C after several decades’ cultivation. This demonstrated that the decline of total soil C pool after grassland conversion was selectively removing microbial residue C. Lower ratio of fungal to bacterial residue C after grassland conversion suggested a shift in the composition of microbial residue C, with potential consequences for changes in soil C quality and stock associated with land cultivation. Collectively, our findings foster the importance of microbial-derived organic C for soil C stock maintenance and emphasize the necessity to take subsurface soils into account when evaluating the role of microbial-derived C to soil C losses under land-use change.

The contribution of microbial residues to soil organic carbon (SOC) is a process highly influenced by soil properties. We evaluated the presence of microbial amino sugar residues in soil (0–50 cm) of control and warmed plots in an alpine meadow on the Qinghai-Tibet Plateau. Alpine grasslands in the Qinghai-Tibet Plateau store large amounts of soil C and are highly vulnerable to climate change. Results showed that warming significantly increased total microbial residues across the 0–50 cm soil depth. The proportion of microbial-derived C to SOC significantly increased in warmed plots (52% on average) by soil depth compared to the control (38%). Higher microbial turnover and selective preservation into organo-mineral complexes likely explains the observed result. Given insignificant change in total SOC, our results infer an alteration of the SOC source configuration (microbial-derived vs. plant-derived). The observed greater magnitude of warming effects on fungal residues compared to bacterial illustrate a distinct community response to warming. We conclude that warming has the potential to influence soil C sequestration through increased microbial residue inputs, consequently altering its composition and source configuration. Our work provides valuable insights at the molecular level to identify mechanisms of microbial-mediated C processes that are influenced by climate change in high elevation ecosystems.

A potentially important factor for destabilization of soil organic C (SOC) in the conversion of ecosystems to arable land is the impact to microbial residues, which have been recently shown to be a significant source of SOC pool. Here, we present the results of a study investigating the changes of microbial residues along a chronosequence of cultivation (5, 15, and 25 years) and attempted restoration (6 and 12 years of agricultural abandonment) in a marsh wetland of China. The wetland cultivation depleted the fraction of SOC derived from microbes, in particular fungal residues. During agricultural abandonment and restoration, bacterial residues accumulated relatively more rapidly than fungal analogs, where fungal residues represented the major microbial SOC pool overall. These differential responses of microbial residues are illustrative of the susceptibility of this SOC pool and indicate their importance as a tool to track soil stability with land-use change. Our work points to the need for future research to focus more strongly on the nature and mechanisms of microbial residue–mediated C process during chronic land-use changes.

Microbial residues are a significant component of soil organic carbon (C), yet their distribution and function remain understudied. We evaluated changes in microbial residues and their contribution to organic C along a soil development sequence on three contrasting parent materials (granite, basalt and andesite) and three conifer biomes (ponderosa pine (Pinus ponderosa Laws.), PP; white fir (Abies concolor Lindl.), WF; and red fir (Abies magnifica A. Murr.), RF) at different elevations in the Sierra Nevada of California. Soil samples were taken from both A and B horizons and microbial residues were determined by amino sugar analysis. The effect of conifer biome on amino sugars was complex and dependent on parent material and horizon. We found parent material significantly influenced soil amino sugars which exhibited a pattern of andesite > basalt > granite in both A and B horizons. Both correlation and redundancy analyses indicated a significant correlation of amino sugars with the amount of short-range-order materials. This suggests soil mineralogy plays an important role in influencing amino sugar accumulation. This is further supported by larger differences among parent materials than between conifer biomes in ratios of fungal-to bacterial-derived amino sugars. The proportion of amino sugars to soil organic C was significantly influenced by parent material in the B horizon following the pattern of basalt > andesite > granite, but not affected by conifer biome. Our results suggest that mineralogy strongly influences the degree to which soil microbial residues persist in temperate forest soils.

To test the impact of a range of long-term land use types on the partitioning of microbial residues among soil particles, samples from a Mollisol with plots under 100 years of continuous arable cropping, 30 years of simulated overgrazing to severely degraded bare soil, or 30 years of grassland restoration were investigated. The microbial residues, which were assessed among three particle-size fractions (

Crop residue incorporation is an effective agricultural management to enhance soil organic carbon and nitrogen (N) sequestration, which inevitably affects the crop N uptake, and consequently the allocation of N in the soil-plant system. However, to what extent the crop residue type and soil fertility moderate the contribution and recovery of maize (Zea mays. L) residue-derived N (residue-N) in the soil-plant system is poorly understood. Therefore, a 2-year in situ experiment was conducted on an Alfisol with high fertility (HF) and low fertility (LF), along with the application of 15N-labeled maize root or straw (both stem and leaf) residues. The results showed that the HF treatment had a smaller proportion of residue-N in total N uptake compared with the LF treatment (on average of 2.0% vs. 3.6%), but with a larger recovery rate (on average of 23.5% vs. 12.8%). The application of straw residues increased the proportion of residue-N in plant total N compared with that from the application of root residues in the HF treatment. The percentage of residue-N in total soil N (TSN) and the residue-N recovery in the LF soil were 60.7–108.5% and 9.6–25.8% higher than those in the HF soil, respectively. The proportion of root-derived N (root-N) in TSN was significantly higher than that of straw-derived N (straw-N) in the LF soil. In addition, total residue-N recovery rates were similar among all residue-amended treatments. The results indicated that the soil with high fertility could increase residue-N utilization by plant, especially the straw-N, while the soil with low fertility favored the accumulation of the residue-N in soil, especially the root-N, and its low fertility was more likely to be improved. Overall, soil fertility and residue type could significantly affect the allocation of maize residue-N in the soil-plant system.

Microbial residues represent a significant soil organic matter pool and participate in soil aggregation. The addition of organic manure is known to modify soil aggregation and strongly influence soil microbial residues. How manure application influences the spatial distribution of microbial residues in soil aggregates is largely unknown. This study attempts to determine the effect of manure application at various rates on the content and distribution of microbial residues among aggregates of different sizes. We used a long-term manure application experiment in a Mollisol in northeastern China, where manure has been applied since 2001 at rates of 0, 7.5, 15, and 22.5 Mg ha−1 yr−1 (dry weight). The abundance of microbial residues was indicated by amino sugar analysis. Glucosamine and muramic acid were used as biomarkers for fungal and bacterial residues, respectively. Amino sugars were examined within four aggregate fractions: large macroaggregate (>2000 μm), small macroaggregate (250–2000 μm), large microaggregate (53–250 μm) and small microaggregate ( 250 μm macroaggregates compared with microaggregates. However, effects of manure application rates on amino sugar accumulation in larger aggregates were limited when manure rate was increased from 15 to 22.5 Mg ha−1 yr−1. The response of fungal- and bacterial-derived amino sugars to manure application rates differed among aggregate fractions, i.e., glucosamine associated with macroaggregates increased more than that of microaggregates, whereas the enhancement of muramic acid was prominent in both macroaggregates and large microaggregates. The mass proportions of macroaggregates and MWD showed significant positive correlations with amino sugar contents, indicating that these microbial residues are involved in the formation and stabilization of aggregates. Manure applications greatly increased the contribution of microbial residues to soil organic C (SOC) in small macroaggregates and large microaggregates (P ≤ 0.05). We conclude that higher manure input may promote soil aggregation and higher SOC storage, which is closely related to a greater microbial residues-mediated improvement of soil aggregate stability. Our results also suggest that measurement of amino sugar content is a useful approach to assess fungal and bacterial contributions to soil aggregation.

Soils simultaneously provide multiple ecosystem functions (i.e. multifunctionality) which are of critical importance in terms of climate regulation and fertility maintenance. Although the influence of tillage practices on many soil functions is well documented, its effect on the resistance and resilience of these functions to climate change from a ‘holistic ecosystem’ view remains poorly understood. In this study, we compared the resistance and resilience to dry-wet cycles, which is predicted to be more frequent and intensified in agricultural soils under climate change, of soil multifunctionality under no-tillage and ridge tillage. We found that no-tillage led to a higher resistance but a lower resilience of soil multifunctionality than ridge tillage in response to dry-wet disturbances. Variation partitioning analysis and mantel correlation between dissimilarity matrices showed that the resistance and resilience of soil multifunctionality was closely related to soil microbial diversity. Soil pH also contributed to the variation in stability of soil multifunctionality, but its explanatory power was much lower than microbial diversity. Our results suggest that tillage practices strongly affect the resistance and resilience of soil multifunctionality to dry-wet cycles, which might exert important consequences for ecosystem services that delivered by agricultural soils under climate change.

The objective of this study was to examine the effects of long-term (18 years) fertilization on contents and distributions of amino sugars within aggregate-size fractions of a clay soil (Udolls, USDA Soil Taxonomy System). Treatments included unfertilized control (CK), mineralization fertilization (NP), and application of mineral fertilizers plus pig manure (NPM). Soil samples were collected from the 0–20-cm layer and fractionated to aggregates of different sizes (>2,000 μm, 250–2,000 μm, 53–250 μm, and 250 μm) of NPM soils were mainly due to a relatively greater accumulation of GluN over MurA, which suggests that accumulation of fungal residues is important for soil aggregation and organic C storage in the tested soil. By contrast, mineral fertilization was ineffective on soil aggregation and amino sugar accumulation. Furthermore, GluN was relatively enriched in fractions of >250 μm while MurA was generally enriched in the 53–250-μm fraction. This indicates that there are specific mechanisms of fungal and bacterial enrichment in different aggregate-size fractions.

Microbial residue represents a significant amount of soil organic matter, and its component amino sugar can serve as time-integrated indicators that reflect chronic effects of agricultural management. We evaluated the influence of different land-use and fertilization treatments on the amounts and patterns of amino sugars (glucosamine and galactosamine) and muramic acid in a Mollisol (Udolls, USDA Soil Taxonomy System) in northeastern China. The treatments included: BL, bareland, without any vegetation; GL, restored grassland which allows plants to re-vegetate naturally; AL, arable land without any fertilizer; ALF, arable land with chemical fertilizer; and ALMF, arable land with chemical fertilizer and pig manure. The amino sugar concentrations differed significantly between various treatments after 26 years, with the order of ALMF > GL > ALF > AL > BL. This suggests that long-term contrasting management changed microbial residue accumulation in soil, which is strongly related to soil organic carbon content. The larger ratios of glucosamine to muramic acid in the GL plots than the AL and BL plots indicated a shift toward fungal-derived residues after 26 years of natural restoration. Our results suggested that different land-use and fertilization treatments clearly influenced amounts and patterns of microbial residues and their contribution to SOM accumulation, primarily due to differences in organic C inputs.

A field experiment was conducted to examine the influences of long-term applications of maize straw and organic manure on carbon dioxide (CO2) emissions from a cultivated Mollisol in northeast China and to evaluate the responses of soil CO2 fluxes to temperature and moisture. Soil CO2 flux was measured using closed chamber and gas chromatograph techniques. Our results indicated that the application of organic amendments combined with fertilizer nitrogen, phosphorus and potassium (NPK) accelerated soil CO2 emissions during the maize growing season, whereas NPK fertilization alone did not impact cumulative CO2 emissions. Cumulative CO2 emissions were higher from soils amended with pig manure relative to those with maize residue. Cumulative CO2 emissions during the growing season were 988 and 1130 g CO2 m−2 under applications of 7500 and 22,500 kg ha−1 pig manure combined with NPK, respectively, which were 42 and 63% higher than the emissions from the control (694 g CO2 m−2). The applications of 2250 and 4500 kg ha−1 maize straw combined with NPK marginally increased soil CO2 emissions by 23 and 28% compared with the control, respectively. A log-transformed multiple regression model including both soil temperature and moisture explained 50–88% of the seasonal variation in soil CO2 fluxes. Cumulative soil CO2 emissions were affected more by applied treatments than by soil temperature and moisture. Our results suggest that the magnitude of the impact of soil amendments on CO2 emissions from Mollisols primarily depends on the type of organic amendments applied, whereas the application rate has limited impacts.

There is increasing evidence that microorganisms participate in soil C sequestration and stabilization in the form of resistant microbial residues. The type of fertilizers influences microbial activity and community composition; however, little is known about its effect on the microbial residues and their relative contribution to soil C storage. The aim of this study was to investigate the long-term impact (21 years) of different fertilizer treatments (chemical fertilizer, crop straw, and organic manure) on microbial residues in a silty clay loam soil (Udolls, USDA Soil Taxonomy). Amino sugars were used to indicate the presence and origin of microbial residues. The five treatments were: CK, unfertilized control; NPK, chemical fertilizer NPK; NPKS1, NPK plus crop straw; NPKS2, NPK plus double amounts of straw; and NPKM, NPK plus pig manure. Long-term application of inorganic fertilizers and organic amendments increased the total amino sugar concentrations (4.4–8.4 %) as compared with the control; and this effect was more evident in the plots that continuously received pig manure (P < 0.05). The increase in total amino sugar stock was less pronounced in the straw-treated plots than the NPKM. These results indicate that the accumulation of soil amino sugars is largely influenced by the type of organic fertilizers entering the soil. Individual amino sugar enrichment in soil organic carbon was differentially influenced by the various fertilizer treatments, with a preferential accumulation of bacterial-derived amino sugars compared with fungal-derived glucosamine in manured soil.

Predicting carbon (C) mineralization of crop residues returned to soils is important for forecasting carbon dioxide (CO2) emissions into the atmosphere and soil nitrogen (N) availability. In this study, a laboratory incubation experiment was conducted to investigate C mineralization of residues of soybean (Glycine max), maize (Zea mays), and their mixture placed on the soil surface and incorporated into the soils in a Mollisol in northeast China. Both the residue type and placement significantly affected C and N mineralization, while no significant interactions between them were observed on cumulative C mineralization. The soybean residue had a higher decomposition rate than the maize residue regardless of their placements; decomposition rates for both soybean and maize residues placed on the soil surface were higher than those of the same residues incorporated into the soils. Moreover, non-additive effects on the contribution of each residue type to C mineralization of the residue mixture were not observed. Our results suggest that crop residue with a low N concentration and a high C/N ratio (such as maize) can be incorporated into soils to immobilize N and decrease CO2 emissions in comparison with crop residue placed on the soil surface.

In this study, the influence of 10 years’ continuous application of organic manure at various rates combined with chemical fertilizer on microbial residues was evaluated in a highly fertile temperate soil. The presence and origin of microbial residues were indicated by amino sugar analysis. The treatments were: (1) CK, unfertilized control; (2) OM0, only chemical fertilizer, no manure added; (3) OM1, organic manure added at 7.5 Mg ha−1 year−1 plus chemical fertilizer; (4) OM2, organic manure added at 15 Mg ha−1 year−1 plus chemical fertilizer; and (5) OM3, organic manure added at 22.5 Mg ha−1 year−1 plus chemical fertilizer. Fertilization significantly increased the total amino sugar concentrations, especially in the plots with higher manure addition rates (OM2 and OM3 plots, P < 0.05). This suggests a positive effect of organic manure combined with chemical fertilizer on the accumulation of microbial residues in soil. However, the highest manure rate (OM3) did not lead to further increase in the total amino sugar pool as compared with the moderate manure rate (OM2). This suggests manure addition “saturates” in its effect on microbial residue build-up. The different patterns of individual amino sugars suggest a change in the quality of microbial-derived soil organic matter after 10 years.

A B S T R A C T Tillage practices affect soil microorganisms, which in turn influence many processes essential to the function and sustainability of soil. In this study, the changes in soil microbial biomass and community composition in response to conventional tillage (CT, moldboard plowing and post-harvest residue removal) and no-tillage (NT) practices were examined during a maize (Zea mays L.) growing season in a clay loam soil (Typic Hapludoll) in northeastern China. Soil samples were taken in May, June, July, August, and September of 2008 at 0–5, 5–10, and 10–20 cm depths. Microbial communities were characterized by phospholipid fatty acid (PLFA) analysis. While microbial biomass increased at the beginning then decreased toward the end of the growing season in CT soils, it showed the opposite trend in NT soils. Microbial community structure showed better distinction among sampling months than between tillage practices. These results suggest that seasonal variations in soil microbial communities could be greater than changes associated with tillage treatments. However, microbial biomass accumulation was tillage dependent. On average, NT treatment resulted in 21% higher microbial biomass in 0–5 cm depth than CT treatment (P < 0.05). Higher fungi to bacteria ratio was also observed under NT than CT treatment at both the 0–5 and 5–10 cm sampling depths. These data demonstrate that examining the effect of management practices on soil quality based on soil microbial communities should consider seasonal changes in the environmental properties. It is strongly recommended that NT practice should be adopted as an effective component of an overall strategy to improve soil quality and sustainability in northeastern China.

It is still unclear that whether substantial amounts of organic manure amendment could increase soil organic carbon (SOC) sequestration in a Chinese Mollisol with relatively high organic C content. Further, changes accompanied by different organic carbon (C) fractions are not well understood based on long-term experiment. The knowledge of this kind is important for assessing the potential for C sequestration of a high SOC soil and selecting effective management practices for increasing soil C sequestration and productivity in agroecosystem of northeastern China. This study was aimed to assess the effects of 10 years’ organic manuring at various rates combined with chemical fertilizer on organic C sequestration in topsoil (0–20 cm) and to identify changes in different SOC (total, labile, and recalcitrant) pools. Five fertilization treatments were included: (1) CK, unfertilized control; (2) OM 0 , only chemical fertilizer, no manure added; (3) OM 1 , organic manure added at 7.5 Mg ha −1 year −1 plus chemical fertilizer; (4) OM 2 , organic manure added at 15 Mg ha −1 year −1 plus chemical fertilizer; and (5) OM 3 , organic manure added at 22.5 Mg ha −1 year −1 plus chemical fertilizer. Chemical fertilizer was supplied with the same rate in each fertilized treatment. We found that the application of graded rates of manure from OM 1 to OM 3 significantly enhanced total SOC, labile C pools, and recalcitrant C pool as compared with OM 0 and CK. The C storage (in top 20 cm) in the OM 0 , OM 1 , OM 2 , and OM 3 was increased by 3.19%, 12.5%, 14.5%, and 18.2%, respectively, over the CK treatment, suggesting that the chemical fertilizer addition had less effects on C-sequestration in topsoil compared to manure plus mineral fertilizer amendment. Moreover, topsoil C-sequestration increased with organic manure addition rates with an order of OM 3 (10.5 Mg ha −1 ) > OM 2 (8.4 Mg ha −1 ) > OM 1 (7.2 Mg ha −1 ). A positive relationship between C sequestration and organic manure input indicates that the soil has not reached its maximum capacity of C sequestration. Application of organic manure with chemical fertilizer was found to produce greater size of both labile and recalcitrant pools than application of mineral fertilizers alone. In most cases, the increases in these C fractions were greater when organic manure was supplied at higher rates. Moreover, increase in recalcitrant C (10.5–29.5%) was significantly higher than labile C (5.6–10.2%) in manure treated plots as compared with no amendment plot. This indicated that a majority of organic C sequestered due to C inputs was accumulated and stabilized in recalcitrant C pool. In general, organic manure combined with inorganic fertilizer exerted greater influence on topsoil C storage and crop yield than chemical fertilizer alone. Based on crop yield and soil C storage, applying organic manure at the rate of 22.5 Mg ha −1 year −1 was the most effective way to improve soil productivity and C sequestration in the agroecosystem of northeastern China. However, taking use efficiency of organic inputs and environmental factors into consideration, application of manure at moderate rate (about 15 Mg ha −1 year −1 ) may be more feasible in this region.

Carbon (C) and/or nitrogen (N) in plant residues can be assimilated into microbial biomass during the plant residue decomposition before incorporation into SOM in the form of microbial residues. Yet, microbial transformation of plant residue-N into microbial residues and the effects of inorganic N inputs on this process have not been well documented. Here, we undertook a 38-week incubation with a silt loam soil amended with a 15N-labeled maize (Zea mays L.) residue to determine how the transformation of maize residue-N into soil amino sugars was affected by rates of inorganic N addition. The newly metabolized amino sugars derived from maize residue-N were differentiated and quantified by using an isotope-based gas chromatography–mass spectrometry technique. We found that greater amounts of maize residue-N were transformed into amino sugars with lower inorganic N addition at the early stages of the plant residue degradation. However, the trend was reversed during later stages of decay as greater percentage of maize residue-N (8.6–9.4%) were enriched in amino sugars in the Nmed and Nhigh soils, as compared with N0 and Nlow (7.5–8.2%). This indicated that higher availability of inorganic N could delay the transformation process of plant-N into microbial residues during the mineralization of plant residues. The dynamic transformations of the plant residue-N into individual amino sugars were compound-specific, with very fast incorporation into bacterial MurAM-new found during the initial weeks, while the dynamics of maize residue-derived GluN exhibited a delayed response to assimilate plant-N into fungal products. The findings indicated differential contributions of maize residue decomposing microorganisms over time. Moreover, we found no preferential utilization of inorganic N over plant residue-N into amino sugars during the incubation course, but inorganic N inputs altered the rate of plant-N accumulation in microbial-derived organic matters. Our results indicated that higher N availability had a positive impact on the accumulation or stabilization of newly-produced microbial residues in the long term.

Agricultural management has significant impacts on soil microorganisms, which in turn influence soil relevant processes including soil organic matter (SOM) turnover. Amino sugars, as a microbial residue biomarker, are highly involved in microbial-mediated SOM cycling. However, the response of microbial-derived amino sugars to changes in soil management practices is poorly characterized in the arable soils (Typic Hapludoll, USDA Soil Taxonomy) in northeast China. The objective of this study was to assess the effects of soil tillage (conventional tillage CT vs. no tillage NT) and crop rotation (continuous corn ( Zea mays L.) monoculture vs. corn–soybean [ Glycine max (L.) Merr.] rotation) on the accumulation of amino sugars in this region. The following hypotheses were tested: (1) in an agroecosystem, tillage and crop rotation influence the accumulation of microbial residues in soils; (2) responses of fungal and bacterial cell wall residues to the practices are different, i.e., fungal-derived cell wall residues more benefited by no-tillage practices than bacterial. Implementation of no-tillage practices for 6 years significantly increased soil total amino sugars with an increase of 59% as compared with conventionally tilled plots ( P −1 soil) occurred in the continuous corn monoculture system as compared with the corn–soybean rotation field (943–1112 mg kg −1 soil) in the soil layers from 0 to −20 cm. Data obtained suggest that NT practices combined with continuous high C inputs can be adopted as an effective strategy to improve the cropland SOM in northeast China. Our results also indicate that it is possible to use amino sugars as an indicator to assess the changes in microbial community within a given ecosystem.