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Soil organic matter (SOM) changes with land use and soil management, yet the controlling factors over the chemical composition of SOM are not fully understood. We applied quantitative 13C nuclear magnetic resonance and spectral editing techniques to measure chemical structures of SOM from different land use types. The land use types included a native grassland (nGL), a crop land with straw burning in the field (bCL), a restored grassland (rGL) and a cropland with straw removed out of the field (rCL) for 28 years. The abundances of O CH groups from carbohydrates were higher in the SOMs of the nGL and rGL than in those of the rCL and bCL, while the abundances of OCH3 and aromatic C O groups from lignin were higher in the SOMs of the three-ever cultivated lands (rGL, rCL and bCL) than in that of the nGL. Although aromatic C C groups were most dominant in the Mollisols, they did not consistently decrease after the burnings of straw were ceased in the fields of the rCL and rGL compared to the bCL with continuous burning. In addition, the COO groups were bound with the aromatic C C groups in all the land use types, and the sizes of the aromatic clusters were affected by the land use types. The labile and recalcitrant components were correlated with SOC contents the mineral-associated and particular SOM in a contrasting way. Our results suggested that the chemical composition of SOM in the Mollisol depended on land use types, and that labile and recalcitrant components might be protected through mineral associations and aggregation, respectively. The most abundant aromatics in the Mollisols might not just be pyrogenic and could be oxidized to different extents, depending on field drainage conditions.

Crop straw is often incorporated with soil tillage to maintain soil organic carbon (SOC). Both the crop straw addition per se and its associated soil structure changes can stimulate SOC decomposition, known as the priming effect. Yet no attempt has been made to isolate their effects. In addition, the priming effect is usually estimated by using uniformly labeled plant litters in the laboratory, and the impacts of non-uniform labeling on the estimation of the priming are poorly understood. The objectives of this study were 1) to isolate the effects of crop straw addition and soil structure changes on SOC decomposition and microbial community composition and 2) to evaluate the effects of the addition of pulse-labeled straw on estimation of the priming effects. The labeled 13C content and its δ13C abundance in the labile fractions of the straw sequentially extracted by ethanol, water and 0.1 M HCl were similar, but were much larger than those in the stable fractions exacted by 0.1 M NaOH. To identify the effects of soil structure changes, the soil texture of a surface soil was manipulated by adding fine sized particles ( 74 μm macroporosity of the texture-manipulated soil (MMS1) increased, causing strong shifts in microbial community composition characterized by phosphorus lipid fatty acid profiling compared to non manipulated soil (NMS1) during a 56-day incubation. The dynamics and total priming effects estimated using the end mixing model (EMM) based on the δ13C abundance in the labeled straw and the improved priming model (PRIM) based on first-order SOC decomposition agree well. Total straw decomposition and total priming effect in the treatment MMS1 were larger than those in the treatment NMS1 by 175% and 170% with the EMM model, respectively. Our findings highlight the importance of understanding abiotic and biotic interactions underlying SOC turnover in the detritusphere of arable ecosystems.

Soil organic matter (SOM) in subsoils stores more than half of terrestrial organic carbon (C), and may sequestrate more C with increasing organic input due to its low C content (or high mineral reactivity) and high chemical stability. Organic inputs can stimulate microbial decomposition of native SOM (known as the priming effect), while being microbially decomposed and transformed into SOM. Yet, microbial controls over these processes and their influence on soil carbon change in soil profile remain elusive because of technical challenge to separate them. We overcame this challenge by employing a novel approach of combining 13C and 12C isotopes with quantitative solid-state 13C nuclear magnetic resonance (NMR). We used soil samples taken from three soil horizons in a Mollisol profile that dominated with fused-ring aromatics for a 43-day incubation. The signal intensities of the most dominant fused-ring aromatics and nonpolar alkyl groups were reduced due to the priming effect following the addition of 12C enriched glucose. Those signal intensities of O-alkyl and nonpolar alkyl groups increased in SOM spectra following the addition of 13C-labeld glucose, demonstrating accumulation of glucose and microbial residues. With the increasing glucose concentration, priming effect estimated using isotopic method and the magnitudes of signal loss estimated using 13C NMR both increased as exemplified for the Ap horizon soil. However, soil organic C content increased only when the added glucose concentration was beyond a previously non-quantified priming saturation threshold (between 36.0 and 100.0 g glucose-C kg−1 SOM-C). The increase of soil organic C was larger in the subsoils than in the topsoil due to lower microbial biomass, higher microbial growth efficiency (MGE) and mineral reactivity, which were related to the reduced priming effect and enhanced accumulation of microbial and glucose residues in the subsoils. The higher MGE in the subsoils agreed with stronger shifts of microbial community composition, characterized by phosphorous lipid fatty acid profiling, with changing glucose concentration during the incubation. Our findings highlighted the importance of priming saturation threshold, microbial mediation and mineral reactivity, but not SOM recalcitrance, in controlling the dynamics of SOM. Our study provided a novel approach to quantify these parameters and understand the controlling factors in relation to different plant types and soil types.

Crop straw is often retained on the ground as mulch or incorporated into soil through tillage practices. These straw management practices may affect straw decomposition rates and subsequently decomposition products due to mixing straw with soil, straw placement depths and experimental sites. Using litterbags containing maize (Zea mays. L) straw only (STO) or straw-soil mixture (SSM), this study aimed to determine the influence of mixing straw with soil and straw placement depths on decomposition rates and decomposition products, and to understand the relation of chemical composition of straw residues with straw decomposition rates. The litterbags were placed at three soil depths (0, 15 and 30 cm) in the fields at two cold sites (Hailun and Harbin), Northeast China, which had similar precipitations but different temperatures. During the 17-month experiment, the decomposition rate constants were higher by 60 %–160 % at a comparable depth in SSM than in STO. The mixing straw with soil was a more important factor in controlling straw decomposition rates than placement depths and experimental sites at the studied region. The relative abundances of O–alkyl C groups in straw residues decreased during the experimental period, even in the winter, while those of aromatic groups increased profoundly. The relative abundances of O–alkyl C groups in straw residues were lower by 0.4 %–6.9 % and those of aromatic groups were higher by 0 %–4.6 % in SSM than in STO. These findings suggest that the conventional litterbag method underestimated straw decomposition rates. Incorporating straw into soil could enhance maize straw decomposition and the formation of more stable soil organic matter (SOM) in the cold high-latitude studied region.

Fresh organic carbon becomes more accessible to deep soil following losses of surface soil and deep intentional incorporation of crop residues, which can cause the priming effect and influence the quality and quantity of SOC in deep soil. This study determined the priming effect due to addition of water-dissolved 13 C-labeled glucose (0.4 g C kg −1 soil) to a soil taken from 1.00 to 1.20 m depth. The changes in chemical compositions of SOC in soils without (G 0 ) and with (G 0.4 ) glucose addition during a 31-d incubation were investigated with solid-state 13 C cross polarization/total sideband suppression ( 13 C-CP/TOSS) and CP/TOSS with dipolar dephasing nuclear magnetic resonance (NMR) techniques. No glucose remained in the soil after 21 days of incubation, with 48% being completely mineralized into CO 2 emission and 52% being incorporated into SOC. The native SOC was decomposed by 0.23% more in G 0.4 than in G 0 . The NMR spectra demonstrated that both labile and recalcitrant organic compounds in SOC changed during the incubation, but in different manners in G 0 and G 0.4 . During the incubation, the -(CH 2 ) n -abundance in G 0 did not change over time, but in G 0.4 it decreased from Day 0 to Day 21 and then increased from Day 21 to Day 31, suggesting shifts of soil microbial communities only in G 0.4 . After the incubation, in G 0 the abundances of ketones/aldehydes and nonpolar alkyl C increased, but those of aromatic C–C and protonated O-alkyl C (OCH) decreased; In G 0.4 , the abundances of NCH and protonated O-alkyl C (OCH) increased, but those of nonpolar alkyl C and nonprotonated aromatic C–O and ketones/aldehydes decreased. Such inconsistent changes in recalcitrant compounds between G 0 and G 0.4 indicated that glucose addition likely primed the decomposition of aromatic C–O and suppressed the formation of ketones/aldehydes. We have demonstrated for the first time that the priming effect of SOC decomposition in the deep soil was involved with larger notable changes in both labile and recalcitrant structures of native SOC due to glucose addition compared with that without glucose addition.

Soil microbial communities may differ with aggregate size as aggregates constitute a complex environment for microorganisms. However, such effects are inconsistent for mature soils and largely unknown at the initial stage of soil formation. By using an eight-year field experiment established on dug parent material (PM) of a Mollisol, our objectives were (1) to examine the effects of aggregate size on soil microbial communities by analyzing phospholipid fatty acids (PLFAs) and (2) to identify the controlling factors of such changes in microbial community structure. The field treatments included two no-tilled soils under perennial crops and four tilled soils under the same cropping system, with or without chemical fertilization and crop residue amendment and were compared to PM and an arable Mollisol (MO) with only chemical fertilization. Total N, soil organic C (SOC), total PLFAs and composition of soil microbial communities were affected by aggregate size and field treatment. Principal component analysis (PCA) of PLFA profiles demonstrated that the microbial community structure was separated by aggregate size classes via PC1 for all studied soils and by field treatment via PC2 in each aggregate size class. The separations via PC1 were driven by fungi, eukaryotes and bacteria that were associated with the >2, 2–0.25 and 0.25–0.053-mm aggregates, respectively. The separations via PC2 were driven by gram positive (G(+)) bacteria and actinomycetes that were associated with the field treatments, being more separated in 2–0.25 mm aggregates than in 0.25–0.053 mm aggregates. These results suggested that the formation of macroaggregates from weathered particles or microaggregates had governed the distribution of microbial functional groups in different sizes of aggregates due to the variations of physical and chemical environment. The variations among field treatments were larger in larger aggregates possibly due to more influence of organic carbon input and tillage on formation of larger aggregates.

A range of agricultural practices influence soil microbial communities, such as tillage and organic C inputs, however such effects are largely unknown at the initial stage of soil formation. Using an eight-year field experiment established on exposed parent material (PM) of a Mollisol, our objectives were to: (1) to determine the effects of field management and soil depth on soil microbial community structure; (2) to elucidate shifts in microbial community structure in relation to PM, compared to an arable Mollisol (MO) without organic amendment; and (3) to identify the controlling factors of such changes in microbial community structure. The treatments included two no-tilled soils supporting perennial crops, and four tilled soils under the same cropping system, with or without chemical fertilization and crop residue amendment. Principal component (PC) analysis of phospholipid fatty acid (PLFA) profiles demonstrated that microbial community structures were affected by tillage and/or organic and inorganic inputs via PC1 and by land use and/or soil depth via PC2. All the field treatments were separated by PM into two groups via PC1, the tilled and the no-tilled soils, with the tilled soils more developed towards MO. The tilled soils were separated with respect to MO via PC1 associated with the differences in mineral fertilization and the quality of organic amendments, with the soils without organic amendment being more similar to MO. The separations via PC1 were principally driven by bacteria and associated with soil pH and soil C, N and P. The separations via PC2 were driven by fungi, actinomycetes and Gram (−) bacteria, and associated with soil bulk density. The separations via both PC1 and PC2 were associated with soil aggregate stability and exchangeable K, indicating the effects of weathering and soil aggregation. The results suggest that in spite of the importance of mineral fertilization and organic amendments, tillage and land-use type play a significant role in determining the nature of the development of associated soil microbial community structures at the initial stages of soil formation.

A field experiment for river restoration was carried out in Chaohe river, a second largest river in Beijing, China. The study site with an area of 2, 800 m2 was set up in the downstream zone of Miyun reservior. Multiple water quality parameters, including pH, TN, TP and COD, were used to evaluate the removal efficiency of the river restoration project. The results showed that pH of water changed in a narrow rang from 6.98 to 7.11 during the investigation period. The pH value had no significant difference between the planted treatment and the non-planted control. From April to October, 2009, TN and TP of water were reduced regularly. At the beginning, the reduction tendency was sharp, then became flat. The mean removal efficiencies of TN and TP were 5.8% and 16%, respectively. No significant difference existed between the TN content of the planted treatment and that of the control. Contrarily, the TP content difference was significant between the two treatments. The maximum removal efficiencies were 30.2% on COD, 7.8% on TN and 25.5% on TP for the aquatic plant-based remediation system. This study suggested that the restoration of aquatic vegetation could potentially be used for mitigating the degradation of water quality and improve water carrying capacity.