秸秆还田下土壤有机质激发效应研究进展

土壤有机质是农田肥力的基础与核心，对作物产量、农业环境，甚至地球碳循环意义重大。作物秸秆作为农田土壤有机碳库的重要外部补充，其还田过程对土壤有机碳周转和碳库平衡具有显著影响。激发效应是一种因新鲜有机质输入而导致土壤本底有机质矿化速率发生改变的现象。秸秆还田导致的土壤有机质分解激发，不仅涉及到秸秆资源化高效利用，还直接关系到农田土壤碳库的平衡及其功能，因此备受科学界关注。尽管对外源有机质输入引起的土壤有机质激发效应的理论研究已取得了较大进展，但如何结合最新的理论结果到秸秆还田固碳减排的生产实践中仍面临着较大的挑战，这主要归结于对农田土壤有机质分解激发效应的发生特点和规律，及其背后的土壤、气候、管理等相关的驱动因子和过程还未完全明确。据此，本文首先对土壤有机质分解激发效应发生的理论研究进展（包括：共代谢理论、氮矿化理论、化学计量比和微生物残体再利用）进行了系统综述。其次，结合已有的研究证据和理论假设进一步概述了秸秆还田过程中影响激发强度和方向的潜在驱动因素，如：秸秆类型和数量、还田方式、水肥管理、土壤属性、气候因子等。最后，从秸秆还田的高效性、农田碳库的可持续和农业环境的友好性出发，对秸秆还田土壤有机质分解激发的潜在研究方向进行了展望，并就秸秆还田改善土壤碳库的优化措施提出了建议。     

秸秆及其生物炭对土壤碳库管理指数及有机碳矿化的影响

以河南省粮食主产区壤质潮土和砂土为研究对象,通过盆栽试验和室内恒温培养试验,研究了生物炭与不同腐殖化程度的传统有机物料(秸秆和腐熟鸡粪)单施及配施对壤质潮土和砂土有机碳储量、活性及碳库管理指数的影响,并进一步比较了小麦秸秆直接还田和制炭还田对土壤有机碳矿化的影响,以及生物炭对土壤原有有机碳矿化的调控作用。结果表明:相同添加量下,生物炭对土壤有机碳含量的提升效果优于秸秆和腐熟鸡粪,在壤质潮土和砂土上分别较对照提升了63.15%和115.62%。另外,生物炭显著增加了土壤稳态碳含量和土壤碳库指数(CPI),但降低了土壤碳素有效率(SC)和碳库活度指数(AI),对土壤易氧化有机碳(POXC)和碳库管理指数(CMPI)无显著影响,添加秸秆显著增加了2种土壤POXC含量、基础呼吸和CPMI。进一步通过室内恒温培养试验发现,秸秆可在培养前期(0～37天)大幅度提升2种类型土壤有机碳矿化速率和累积矿化量,秸秆制炭还田对土壤有机碳矿化无显著影响。此外生物炭对土壤原有有机碳矿化的调控作用受其施用量、外源活性有机碳输入和土壤类型的影响,高量生物炭(2%)对非秸秆还田土壤有机碳矿化表现出较强的负激发效应,而低量生物炭(0.55%)对秸秆还田土壤有机碳矿化表现出较明显的负激发效应。因此,从"固碳减排"角度考虑,秸秆制炭还田是更合理的利用方式,且应根据土壤施肥管理措施和土壤类型考虑生物炭的施用量,添加质量比为2%的生物炭可显著抑制土壤原有有机碳矿化,降低CO_2排放,但应避开秸秆快速腐解期施用。     

Nitrogen and phosphorus constrain labile and stable carbon turnover in lowland tropical forest soils

Tropical forests contain a large stock of soil carbon, but the factors that constrain its mineralization remain poorly understood. Microorganisms, when stimulated by the presence of new inputs of labile organic carbon, can mineralize (‘prime’) soil organic matter to acquire nutrients. We used stable carbon isotopes to assess how nutrient demand and soil properties constrain mineralization of added labile (sucrose) carbon and pre-existing (primed) soil carbon in tropical forest soils. In a series of lowland tropical forest soils from Panama, we found that the mineralization of fresh labile carbon was accelerated foremost by phosphorus addition, whereas the mineralization of pre-existing soil carbon was constrained foremost by nitrogen addition. However, there was variation in the relative importance of these nutrients in different soils and the largest effects on the acceleration of sucrose metabolism and constraint of priming occurred following the addition of nitrogen and phosphorus together. The respiration responses due to sucrose or primed soil carbon mineralization were reduced at pH below 4.8 and above 6.0. We conclude that in these tropical forest soils, phosphorus availability is more important in promoting microbial mineralization of sucrose carbon, whereas nitrogen availability is more important in constraining the priming of pre-existing soil organic carbon. This response likely arises because nitrogen is more closely coupled to organic matter cycling, whereas phosphorus is abundant in both organic and inorganic forms. These results suggest that the greatest impact of priming on soil carbon stocks will occur in moderately acidic tropical forest soils of low nitrogen availability. Given long-term changes in both atmospheric carbon dioxide and nitrogen deposition, the impact of priming effects on soil carbon in tropical forest soils may be partially constrained by the abundance of nitrogen.     

Influence of moisture on the stability of soil organic matter and plant residues

The effect of three levels of soil moistening on the organic matter mineralization was assessed for three arable soils and wheat straw in the course of a 150-day-long incubation experiment. It was found that the intensity of the organic matter mineralization increased in parallel to soil moistening in the podzolized chernozem and dark-chestnut soil and remained stable in the gray forest soil, which was explained by the low content of easily mineralizable fractions of active organic matter in the latter soil. The mineralization of wheat straw depended on the soil moistening rather than on soil properties.     

Long-term changes in land use impact the accumulation of microbial residues in the particle-size fractions of a Mollisol

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 (<2, 2–20, and 20–250 μm) by amino sugar extraction, exhibited change with particle size and land use. Converting arable cropping to bare soil induced substantial depletion of amino sugars associated with the clay-size fraction, as a proportion of total carbon (C) and total soil mass, but not the silt- and sand-size fractions. Alternatively, switching arable soil to grassland increased amino sugar stocks in both the clay- and sand-size fractions. Analysis of the relative input of fungal and bacterial derived amino sugars indicated that fungal sources are the most dynamic with respect to land use change. These results highlight the selective vulnerability of microbial C pools in finer fractions under low plant C input and the selective recovery in specific fractions upon restoration, emphasizing the importance of the conversion of plant organic matter into mineral-associated microbial residues to promote stable soil organic C.     

Pedotransfer functions for the pool size of slowly mineralizable organic N in sandy arable soils

The major aim of this study was to evaluate how the pool size of slowly mineralizable, ‘old’ soil organic N can be derived from more easily accessible soil and site information via pedotransfer functions (PTF). Besides modeling, this pool size might be of great importance for the identification of soils with high mineralization potential in drinking‐water catchments. From long‐term laboratory incubations (ca. 200 days) at 35 °C, the pool sizes of easily mineralizable organic N (N_fast), mainly in fresh residues, and slowly mineralizable, ‘old’ soil organic N (N_slow) as well as their first‐order rate coefficients were obtained. 90 sandy arable soils from NW Germany served to derive PTFs for N_slow that were evaluated using another 20 soils from the same region. Information on former land‐use and soil type was obtained from topographical, historical, and soil maps (partly from 1780). Pool size N_slow very strongly depends on soil type and former land‐use. Mean pool sizes of N_slow were much lower in old arable lowland (105 mg N kg^–1) than upland soils (175 mg N kg^–1) possibly due to lower clay contents. Within lowlands, mean pool sizes in former grassland soils (245 mg N kg^–1) were 2 to 3 times larger than in old arable soils due to accumulation of mineralizable N. In contrast, mean pool sizes of N_slow were lowest in recently cleared, former heath‐ and woodland (31 mg N kg^–1) as a result of the input of hardly decomposable organic matter. Neither N nor C in the light fraction (density < 1.8 g cm^–3) was adequate to derive pool size N_slow in the studied soils (r² < 0.03). Instead, N_slow can be accurately (r² = 0.55 – 0.83) derived from one or two basic soil characteristics (e.g. organic C, total N, C : N, mineral fraction < 20 μm), provided that sites were grouped by former land‐use. Field mineralization from N_slow during winter (independent data set) can be predicted as well on the basis of N_slow‐values calculated from PTFs that were derived after grouping the soils by former land‐use (r² = 0.51***). In contrast, using the PTF without soil grouping strongly reduced the reliability (r² = 0.16).     

The priming effect of organic matter: a question of microbial competition?

It is generally accepted that the low quality of soil carbon limits the amount of energy available for soil microorganisms, and in turn the rate of soil carbon mineralization. The priming effect, i.e. the increase in soil organic matter (SOM) decomposition rate after fresh organic matter input to soil, is often supposed to result from a global increase in microbial activity due to the higher availability of energy released from the decomposition of fresh organic matter. Work to date, however, suggests that supply of available energy induces no effect on SOM mineralization. The mechanisms of the priming effect are much more complex than commonly believed. The objective of this review was to build a conceptual model of the priming effect based on the contradictory results available in the literature adopting the concept of nutritional competition. After fresh organic matter input to soils, many specialized microorganisms grow quickly and only decompose the fresh organic matter. We postulated that the priming effect results from the competition for energy and nutrient acquisition between the microorganisms specialized in the decomposition of fresh organic matter and those feeding on polymerised SOM.     

The effect of maize straw placement on mineralization of C and N in soil particle size fractions

Fundamental knowledge about the complex processes during the decomposition, mineralization and transfer of residue organic matter in soils is essential to assess risks of changes in agricultural practices. In a double tracer (¹³C, ¹⁵N) experiment the effect of maize straw on the mineralization dynamics and on the distribution of maize-derived organic matter within particle size fractions was investigated. Maize straw (a C₄ plant) labelled with ¹⁵N was added to soils (13.2 g dry matter kg^–1 soil) which previously had grown only C₃ plants, establishing two treatments: (i) soil mixed with maize straw (mixed), and (ii) soil with maize straw applied on the surface (surface). Samples were incubated in the laboratory at 14°C for 365 days. The size fractions (> 200 μm, 200–63 μm, 63–2 μm, 2–0.1 μm and < 0.1 μm), obtained after low-energy sonication (0.2 kJ g^–1), were separated by a combination of wet-sieving and centrifuging. The mineralization of maize C was similar in the two treatments after one year. However, decomposition of maize particulate organic matter (predominantly in the fraction > 200 μm) was significantly greater in the mixed treatment, and more C derived from the maize was associated with silt- and clay-sized particles. A two-component model fitted to the data yielded a rapidly mineralizable C pool (about 20% of total C) and a slowly mineralizable pool (about 80%). Generally, the size of the rapidly mineralizable C pool was rather small because inorganic N was rapidly immobilized after the addition of maize. However, the different mean half-lives of the C pools (rapidly decomposable mixed 0.035 years, and surface-applied 0.085 years; slowly decomposable mixed 0.96 years, and surface-applied 1.7 years) showed that mineralization was delayed when the straw was left on the surface. This seems to be because there is little contact between the soil microflora and plant residues. Evidently, the organic matter is more decomposed and protected within soil inorganic compounds when mixed into the soil than when applied on the soil surface, despite similar rates of mineralization.     

Carbon mineralization and properties of water-extractable organic carbon in soils of the south Loess Plateau in China

Addition of organic manure over thousands of years has resulted in the development of very fertile soils in parts of the Loess Plateau in Northwest China. This region also suffers from serious soil erosion. For that reason, afforestation of arable soils has taken place. The dynamics of soil organic matter in these soils affected by a very specific management and by land use changes is largely unknown. Therefore, we measured C mineralization in a 35-days incubation experiment and analyzed amounts and properties of water-extractable organic carbon (WEOC) in 12 topsoils of this region. The soils differed in land use (arable vs. forest) and in amounts of added organic manure. Afforestation of arable soils resulted in a distinct stabilization of organic C as indicated by the smallest C mineralization (0.48 mg C g⁻¹ C d⁻¹) and the highest C content (2.3%) of the studied soils. In the soils exposed to intensive crop production without regular addition of organic manure we found the largest C mineralization (0.85 mg C g⁻¹ C d⁻¹) and the lowest contents of organic C (0.9%). Addition of organic manure over a time scale of millennia resulted in high organic C contents (1.8%) and small C mineralization (0.55 mg C g⁻¹ C d⁻¹). The content of WEOC reflected differences in C mineralization between the soils quite well and the two variables correlated significantly. Water-extractable organic C decreased during C mineralization from the soil illustrating its mainly labile character. Carbon mineralization from soils was particularly large in soils with small specific UV absorbance of WEOC. We conclude that amounts and properties of WEOC reflected differences in the stability of soil organic C. Both afforestation of arable land and the long-term addition of organic manure may contribute to C accumulation and stabilization in these soils.     

Composition of organic matter in a subtropical Acrisol as influenced by land use, cropping and N fertilization, assessed by CPMAS 13C NMR spectroscopy

We know much about the influence of management on stocks of organic matter in subtropical soils, yet little about the influence on the chemical composition. We therefore studied by CPMAS ¹³C NMR spectroscopy the composition of the above-ground plant tissue, of the organic matter of the whole soil and of silt- and clay-size fractions of the topsoil and subsoil of a subtropical Acrisol under grass and arable crops. Soil samples were collected from three no-till cropping systems (bare soil; oats−maize; pigeon pea + maize), each receiving 0 and 180 kg N ha⁻¹ year⁻¹, in a long-term field experiment. Soil under the original native grass was also sampled. The kind of arable crops and grass affected the composition of the particulate organic matter. There were no differences in the composition of the organic matter in silt- and clay-size fractions, or of the whole soil, among the arable systems. Changes were observed between land use: the soil of the grassland had larger alkyl and smaller aromatic C contents than did the arable soil. The small size fractions contain microbial products, and we think that the compositional difference in silt- and clay-size fractions between grassland and the arable land was induced by changes in the soil's microbial community and therefore in the quality of its biochemical products. The application of N did not affect the composition of the above-ground plant tissue nor of the particulate organic matter and silt-size fractions, but it did increase the alkyl C content in the clay-size fraction. In the subsoil, the silt-size fraction of all treatments contained large contents of aromatic C. Microscopic investigation confirmed that this derived from particles of charred material. The composition of organic matter in this soil is affected by land use, but not by variations in the arable crops grown.     

Negative priming effect on mineralization in a soil free of vegetation for 80 years

The priming effect (PE) is a complex process corresponding to a modification of mineralization rates of soil organic matter (SOM) following inputs of fresh organic matter (FOM). The priming effect can be either positive or negative (i.e. an acceleration or retardation of SOM decomposition) and is controlled by several factors such as microbial community composition, SOM chemical structure and nutrient availability. The first objective of our experiment was to study negative or positive PE of stabilized SOM. The second was to identify the role of FOM decomposers in the PE of stabilized SOM. We incubated, for 39 days, a fallow soil free of vegetation for 80 years amended with ¹³C‐cellulose and inoculated with a FOM‐decomposing community. The soil contained stabilized SOM. The PE of the stable organic matter was always negative and tended to be more negative when the FOM‐decomposing community was added. This suggests that for this particular soil, SOM mineralization was not limited by energy. Moreover, as the inoculation of a FOM‐decomposing community led to a more negative PE, we assume that the FOM‐decomposing community facilitated the access of FOM to the indigenous bare soil community.     

中国东部主要农田土壤有机碳库的平衡与趋势分析

根据田间测定的有机物料腐殖化系数和土壤有机碳矿化量结果 ,以及统计测算的有机物质进入量 ,分析了中国东部主要农业区农田土壤有机碳库的平衡状况 ,并对其作为大气CO2 的源汇功能进行了初步评价。结果表明 ,各农业区农田土壤中有机物料腐殖化系数的变化趋势为 :松嫩平原 (作物秸秆 0 3 7,作物根 0 47,绿肥 0 2 6 ,粪肥 0 5 4 ) >红壤丘陵区(相应值为 0 2 8,0 45 ,0 2 5 ,0 42 ) >下辽河平原 (相应值 0 3 5 ,0 3 9,0 2 2 ,0 3 3 ) >黄淮海平原(相应值 0 2 5 ,0 3 2 ,0 1 8,0 2 7)。各区间的腐殖化系数并不随气候而呈规律性变异 ,而主要是环境因子综合作用的结果 ,特别是土壤性质有显著影响。现实有机物质进入量条件下 ,各农业区农田土壤有机碳年形成量分别为 :松嫩平原 5 3 8 0kghm- 2 ,下辽河平原 6 79 7kghm- 2 ,黄淮海平原 76 0 7kghm- 2 ,红壤丘陵区 1 0 5 9 7kghm- 2 ;农田土壤有机碳年矿化量分别为 :松嫩平原 90 5 3kghm- 2 ,下辽河平原 72 3 5kghm- 2 ,黄淮海平原 72 3 1kghm- 2 ,红壤丘陵区 782 9kghm- 2 。因此 ,松嫩平原黑土的有机碳库处于亏缺状态 ,土壤有机碳含量还将下降 ,并向大气释放CO2 ;红壤水稻土的有机碳库处于盈余状态 ,土壤有机碳含量还将不断提高 ,是大气C     

Positive and negative carbon mineralization priming effects among a variety of biochar-amended soils

Pyrogenic carbon (biochar) amendment is increasingly discussed as a method to increase soil fertility while sequestering atmospheric carbon (C). However, both increased and decreased C mineralization has been observed following biochar additions to soils. In an effort to better understand the interaction of pyrogenic C and soil organic matter (OM), a range of Florida soils were incubated with a range of laboratory-produced biochars and CO₂ evolution was measured over more than one year. More C was released from biochar-amended than from non-amended soils and cumulative mineralized C generally increased with decreasing biomass combustion temperature and from hardwood to grass biochars, similar to the pattern of biochar lability previously determined from separate incubations of biochar alone.The interactive effects of biochar addition to soil on CO₂ evolution (priming) were evaluated by comparing the additive CO₂ release expected from separate incubations of soil and biochar with that actually measured from corresponding biochar and soil mixtures. Priming direction (positive or negative for C mineralization stimulation or suppression, respectively) and magnitude varied with soil and biochar type, ranging from −52 to 89% at the end of 1 year. In general, C mineralization was greater than expected (positive priming) for soils combined with biochars produced at low temperatures (250 and 400 °C) and from grasses, particularly during the early incubation stage (first 90 d) and in soils of lower organic C content. It contrast, C mineralization was generally less than expected (negative priming) for soils combined with biochars produced at high temperatures (525 and 650 °C) and from hard woods, particularly during the later incubation stage (250-500 d). Measurements of the stable isotopic signature of respired CO₂ indicated that, for grass biochars at least, it was predominantly pyrogenic C mineralization that was stimulated during early incubation and soil C mineralization that was suppressed during later incubation stages. It is hypothesized that the presence of soil OM stimulated the co-mineralization of the more labile components of biochar over the short term. The data strongly suggests, however, that over the long term, biochar-soil interaction will enhance soil C storage via the processes of OM sorption to biochar and physical protection.     

Input related microbial carbon dynamic of soil organic matter in particle size fractions

This paper investigated the flow of carbon into different groups of soil microorganisms isolated from different particle size fractions. Two agricultural sites of contrasting organic matter input were compared. Both soils had been submitted to vegetation change from C3 (Rye/Wheat) to C4 (Maize) plants, 25 and 45 years ago. Soil carbon was separated into one fast-degrading particulate organic matter fraction (POM) and one slow-degrading organo-mineral fraction (OMF). The structure of the soil microbial community were investigated using phospholipid fatty acids (PLFA), and turnover of single PLFAs was calculated from the changes in their ¹³C content. Soil enzyme activities involved in the degradation of carbohydrates was determined using fluorogenic MUF (methyl-umbelliferryl phosphate) substrates.We found that fresh organic matter input drives soil organic matter dynamic. Higher annual input of fresh organic matter resulted in a higher amount of fungal biomass in the POM-fraction and shorter mean residence times. Fungal activity therefore seems essential for the decomposition and incorporation of organic matter input into the soil. As a consequence, limited litter input changed especially the fungal community favoring arbuscular mycorrhizal fungi. Altogether, supply and availability of fresh plant carbon changed the distribution of microbial biomass, the microbial community structure and enzyme activities and resulted in different priming of soil organic matter.Most interestingly we found that only at low input the OMF fraction had significantly higher calculated MRT for Gram-positive and Gram-negative bacteria suggesting high recycling of soil carbon or the use of other carbon sources. But on average all microbial groups had nearly similar carbon uptake rates in all fractions and both soils, which contrasted the turnover times of bulk carbon. Hereby the microbial carbon turnover was always faster than the soil organic carbon turnover and higher carbon input reduced the carbon storage efficiency from 51% in the low input to 20%. These findings suggest that microbial community preferentially assimilated fresh carbon sources but also used recycled existing soil carbon. However, the priming rate was drastically reduced under carbon limitation. In consequence at high carbon availability more carbon was respired to activate the existing soil carbon (priming) whereas at low carbon availability new soil carbon was formed at higher efficiencies.     

养分化学计量比对稻田土壤葡萄糖矿化及其激发效应的影响

选择13C-葡萄糖作为稻田土壤典型易利用态外源有机碳,通过室内培养试验,研究不同C/N/P/S计量比条件下,葡萄糖分解矿化的动态规律及其激发效应。结果表明,稻田土壤中葡萄糖-碳(C)快速矿化,60 d培养实验后,有65.5%~74.6%的葡萄糖-C矿化。养分元素的添加使土壤中葡萄糖-C快速转化碳库的比例逐渐由58%增加至65%,从而使葡萄糖-C矿化率提高了3.9%~12.5%,养分元素的添加量与葡萄糖-C快速转化碳库的比例和矿化率均表现出显著的正相关关系(R2=0.63,p0.05;R2=0.83,p0.05)。葡萄糖-C矿化过程中,导致稻田土壤碳的累积负激发效应为-370~-570 mg kg-1,养分元素添加比例越大,其负激发效应越强,二者呈显著的负相关性(R2=0.66,p0.05)。研究表明,稻田土壤中易利用态碳的矿化受C/N/P/S元素计量比的影响,高比例养分元素的添加,促进土壤中易利用态碳的矿化,抑制土壤原有有机质的分解,增强负激发效应。本研究可为深入了解稻田生态系统碳循环、实现农田土壤肥力提升和温室气体减排提供理论依据。     

钾钠离子添加对土土壤团聚体和有机碳矿化的影响

  【目的】  评价单价阳离子对团聚体稳定性和土壤有机碳矿化的影响,为有机肥尤其是粪肥的科学施用提供理论依据。  【方法】  采用恒温培养法,研究了不同钾、钠离子浓度下土壤团聚体的稳定性。供试土壤为土。试验中设置K+梯度分别为120、200、280、370、540 mg/kg;Na+梯度分别为90、180、270 mg/kg;并设置在K+ 370 mg/kg浓度下,添加Na+ 0、90、180、270 mg/kg的各处理;并以不添加钾钠离子的处理为对照。土壤在70%田间持水量下培养,定期取样,直到培养105天结束。监测土壤有机碳的矿化动态,以双库指数模型模拟有机碳的矿化特征,湿筛法测定团聚体稳定性。  【结果】  所有处理中2～0.25 mm和 >2 mm团聚体的比例均显著高于0.25～0.053 mm团聚体和<0.053 mm粉粘粒,分别占总量的42.0%～52.7%和26.0%~38.9%。添加K+、Na+以及同时添加K+和Na+总体上降低了>2 mm团聚体比例,增加了2～0.25 mm团聚体和< 0.053 mm粉黏粒组分比例,从而降低了团聚体平均重量直径(MWD)和几何平均直径(GMD),钾钠共同作用效果更明显。K+ 120 mg/kg和Na+ 180 mg/kg两个处理显著降低了有机碳累积矿化量,其余单独添加K+或Na+的处理均无显著影响。钾钠共同处理的土壤有机碳累积矿化量随Na+浓度的增大而减小,降幅为1.2%～22.3%。双库指数模型能较好的模拟有机碳的矿化动态。拟合结果表明,所有处理均增加了来源于活性碳库有机碳的矿化量(Ca),降低了除K+540 mg/kg处理外其他处理源于惰性碳库有机碳的矿化量(Cs),且均降低了活性碳库的矿化速率常数(Ka)。同时,所有处理均增加了Ca占总有机碳矿化量的比例(Ra),而降低了Cs的占比(Rs)。冗余分析(RDA)表明,有机碳累积矿化量与GMD显著正相关,与土壤结构稳定性阳离子比值(CROSS)、钠吸附比(SAR)和可交换性钠百分比(ESP)显著负相关。  【结论】  钾、钠离子的累积显著影响了土壤团聚体组成,降低了团聚体MWD和GMD,抑制了惰性有机碳的矿化,提高了活性有机碳的矿化量,这可能是施用钾肥很难提高土壤有机质含量的一个原因。     

Priming effect and C storage in semi-arid no-till spring crop rotations

Adoption of less invasive management practices, such as no-till (NT) and continuous cropping, could reduce CO₂ emissions from agricultural soils by retaining soil organic matter (SOM). We hypothesized that C storage increases as cropping intensity increases and tillage decreases. We also hypothesized that pulsed addition of C increases the mineralization of native SOM. We evaluated C storage at the 0- to 5-cm depth in soils from four crop rotations: winter wheat-fallow, spring wheat-chemical fallow, continuous hard red spring wheat, and spring wheat-spring barley on a Ritzville silt loam (Calcidic Haploxeroll). In two incubation studies using ¹⁴C-labeled wheat straw, we traced the decomposition of added residue as influenced by (1) cropping frequency, (2) tillage, and (3) pulsed additions of C. Differences in ¹⁴C mineralization did not exist among the four rotations at any time throughout the incubations. However, differences in total CO₂ production between the continuous wheat rotations and the fallow rotations point to a priming of native SOM, the degree of which appears to be related to the relative contributions of fungi and bacteria to the decomposition of added residue. Addition of non-labeled wheat straw to select samples in the second incubation resulted in a flush of ¹⁴C-CO₂ not seen in the controls. This priming effect suggests C inputs have a greater effect on mineralization of residual C compared to disturbance and endogenous metabolism appears to be the source of primed C, with priming becoming more pronounced as the fungal:bacterial ratio in the soil increases.     

Decomposition of maize straw in three European soils as revealed by DRIFT spectra of soil particle fractions

Diffuse Reflectance Infrared Fourier Transform (DRIFT) technique was used to investigate decomposition of maize straw residues in particle-size fractions of three European soils in a 1-year incubation experiment that simulated both incorporation and mulching practices. The aliphatic bands in the 2920–2860 cm⁻¹ regions were used to derive decomposition curves and mineralization rates of maize residues. Evaluation of soil organic matter changes by DRIFT in particle-size fractions depended on soil texture and soil aggregate stability. Decomposition could be followed over the full incubation period only in three size-fractions of the silty German soil because of its intermediate aggregate stability. DRIFT spectra were not useful in the least stable sandy Danish soil and in the most stable clayey Italian soil. Decomposition rates derived from DRIFT results were similar to those obtained by isotopic techniques. DRIFT spectroscopy may represent a rapid and accurate method to follow fresh organic matter degradation directly in soil matrix or particle-size fractions in some soils provided that adequate soil dispersion is obtained.     

Soil‐organic‐matter stability in sandy cropland soils is related to land‐use history

Sandy cropland soils in NW Europe were found to contain unusually high organic‐carbon (OC) levels, and a link with their land‐use history has been suggested. This study's aim was to assess the discriminating power of physical and chemical fractionation procedures to yield information on soil‐organic‐matter (OM) stability for these soils. In relict‐ and cultivated‐heathland soils, much higher proportions of 6% NaOCl treatment–resistant but 10% HF–soluble OC (MOC) and N (32.2% and 29.9%) were measured compared to a set of “permanent"‐cropland soils without a history of heathland land use (11.9% and 8.5%). Also, the proportions of 6% NaOCl– and 10% HF treatment–resistant OC and N in the relict and cultivated heathlands (19.2% and 12.0%) were higher than in the permanent‐cropland soils (17.7% and 5.7%). Stepwise multiple linear‐regression yielded a significant relationship between the annual mineralization (g C [100 g OC]^–1), soil OC (g C kg^–1) content, and %MOC: Annual mineralization = 4.347 – 0.087 soil OC – 0.032 %MOC (R² = 0.65). Combinations of incubation experiments for quantification of the labile soil OM pool with chemical fractionation may thus yield meaningful data for development of soil‐organic‐matter models with measurable pools, but their applicability will be limited to specific combinations of former land use with soil, climate, and current management.     

Modeling consequences of straw residues export on soil organic carbon

Cereal straw, which is most often returned to the soil in arable cropping systems, is of renewed interest as a potential source of bioenergy. However, the sustainability of this practice which implies systematic removal of aerial biomass of cereal crops is a controversial issue, particularly in soils having a low soil organic carbon (SOC) content. This study aims at evaluating a simple model (AMG) to predict the consequences of straw export on SOC evolution in various cropping and pedoclimatic conditions. The model was tested on nine long-term field experiments (18–35 yr) dominated by cereal crops and differing in climate, soil type and carbon inputs. The model was able to provide satisfactory simulations of the evolution of SOC in most experiments with a unique set of parameters. The sensitivity analysis indicated that the quality of fit was very sensitive to humification coefficient, moderately sensitive to the size of the stable SOC pool and weakly affected by the ratio of belowground: aerial C input. The dependence of model parameters (humification and mineralization rates) on pedoclimatic conditions (soil clay content and temperature) was analyzed and compared to those proposed in other models (DAISY, CENTURY, ROTHC, CN-SIM) since they vary widely between models. AMG functions provided the best fit in seven out of nine experiments. More generally, the best fit was obtained by assuming that clay content had a small or no effect on humification coefficient and a marked effect on mineralization rate, in accordance with incubation studies in literature. The AMG model was used to simulate the impact of a straw export scenario in nine experiments considering a systematic straw removal one year out of two. With this scenario, straw removal vs. incorporation would reduce carbon stocks by 2.5–10.9% of the initial SOC after 50 yr, depending primarily on the experiment (soil, climate, productivity) and secondarily on the size of the stable C pool (varying from 10% to 65%).