Organic input quality is more important than its quantity: C turnover coefficients in different cropping systems

Annual C input to soil is a major factor affecting soil organic carbon (SOC) dynamics. However different types of C-sources can have different behaviour, in relation to their chemical characteristics and how they interact with soil. Root-derived C, in particular, should be more efficient than other organic materials as a result of the physicochemical and biological characteristics of the surrounding environment, leading to a reduction in the C decomposition rate.To test this hypothesis, we considered a long-term experiment underway in Northern Italy since 1962, comparing permanent meadow and 6 different crop rotations over a wide range of nutrient inputs, in both organic and inorganic forms. C inputs from amendments were measured and those from crops were calculated using allometric functions and crop and residues yields. The time evolution of SOC was studied through a single-pool, first-order kinetic model, allowing the estimation of humification coefficients for residues, roots, farmyard manure and cattle slurries.The highest value of the humification coefficient was estimated for farmyard manure, which confirmed its high efficiency in stabilising SOC content. Root C presented a humification coefficient 1.9 times higher than above-ground plant materials while slurries were intermediate, with a humification coefficient roughly half that of farmyard manure and even lower that of roots.The quality of C input thus seems of fundamental importance for evaluating the sustainability of different cropping systems in terms of SOC dynamics.     

Greater humification of belowground than aboveground biomass carbon into particulate soil organic matter in no-till corn and soybean crops

Quantifying the amount of carbon (C) incorporated from decomposing residues into soil organic carbon (C_S) requires knowing the rate of C stabilization (humification rate) into different soil organic matter pools. However, the differential humification rates of C derived from belowground and aboveground biomass into C_S pools has been poorly quantified. We estimated the contribution of aboveground and belowground biomass to the formation of C_S in four agricultural treatments by measuring changes in δ¹³C natural abundance in particulate organic matter (C_POM) associated with manipulations of C₃ and C₄ biomass. The treatments were (1) continuous corn cropping (C₄ plant), (2) continuous soybean cropping (C₃), and two stubble exchange treatments (3 and 4) where the aboveground biomass left after the grain harvest was exchanged between corn and soybean plots, allowing the separation of aboveground and belowground C inputs to C_S based on the different δ¹³C signatures. After two growing seasons, C_POM was primarily derived from belowground C inputs, even though they represented only ∼10% of the total plant C inputs as residues. Belowground biomass contributed from 60% to almost 80% of the total new C present in the C_POM in the top 10 cm of soil. The humification rate of belowground C inputs into C_POM was 24% and 10%, while that of aboveground C inputs was only 0.5% and 1.0% for soybean and corn, respectively. Our results indicate that roots can play a disproportionately important role in the C_POM budget in soils. Keywords Particulate organic matter; root carbon inputs; carbon isotopes; humification rate; corn; soybean.     

不同磷肥用量对水稻土有机碳矿化和细菌群落多样性的影响

以水稻田田间定位试验为研究对象,利用三维荧光光谱技术(3D EEMs)和454测序技术,采集4个施磷水平(0、30、60、90 kg hm-2a-1)土壤,测定其有机碳矿化、溶解有机碳(DOC)组成和结构特征及细菌群落结构和丰度的变化。结果显示:水田施磷增加了土壤速效磷(Olsen-P),从而提高了土壤DOC含量,加速了有机碳的矿化速率和累积矿化量。3D EEMs结果表明,施磷分别显著增加了荧光指数和鲜度指数值1%~10%和3%~21%,而降低了腐殖化指数,且与土壤生化性质(Olsen-P、DOC和β-葡萄糖苷酶)具有显著相关性。说明施磷通过提高Olsen-P,促进了微生物源DOC的生成,同时降低了DOC的芳香化程度、分子量及腐殖化程度,从而提高了DOC生物可降解性。同时,施磷提高了细菌群落的丰度和多样性,特别是磷诱导了多种具有碳降解功能细菌的增加,从而加速了复杂有机碳的降解和甲烷氧化。此外,主成分分析表明稻田磷素施用量在30~60 kg hm-2a-1时对土壤有机碳矿化及细菌群落多样性的提高作用最为明显。因此,适度施磷能显著提高涉碳降解微生物的活性,从而提高DOC的生物可降解性,加速有机碳的矿化速率,促进稻田土壤有机碳循环。     

栗钙土有机物料的腐解特征及土壤有机质调控

采用砂滤管田间模拟方法,对不同有机物料在冀西北高寒半干旱区栗钙土中的转化特点及控制因素进行了研究。结果表明：（１）有机物（肥）料在栗钙土中初期前３个月分解较快,矿化碳累积量大,此后逐渐变慢,至腐解２年时所有有机物（肥）料渐趋稳定状态．不同有机物（肥）料的分解速率表现为：猪粪＞羊粪＞牛粪＞马粪＞农家肥（土圈粪）．秸秆类有机物料以小麦秸分解最慢,其它则无明显差异．至腐解２年时所有物（肥）料渐趋稳定状态．（２）栗钙土中有机物（肥）料的腐殖化系数变动在０．２５～０．７２之间,并表现为农家肥＞马粪＞牛粪＞羊粪＞猪粪;小麦秸＞莜麦秸＞披碱草;豆科秸秆十分相近．土壤有机质矿化率在ｌ．１０％～４．５３％范围内,坡梁栗钙土为２．８６～４．５３％,旱滩草甸栗钙土为ｌ．１０～３．７３％．（３）应用土壤有机质平衡定量施肥模式,计算了栗钙土为维持中低产和高产土壤有机质平衡所需的平均临界有机肥年施用量．     

不同氧气条件对玉米秸秆在土壤中腐殖化的影响

研究了实验室模拟条件下添加玉米秸秆对土壤中各有机质组分数量的影响以及不同氧气体积分数对玉米秸秆在土壤中腐殖化进程的影响.结果表明:添加玉米秸秆对土壤有机碳(SOC)、水溶性有机碳(wSOC)、胡敏酸(HA)和富里酸(FA)数量的增加有明显作用;高氧和低氧环境均有利于玉米秸秆在土壤中腐殖化的进行;高氧条件有利于腐殖化初期FA的生成,但持续高氧环境不利于FA的积累;在腐殖化后期,高氧条件使FA大量转化为HA并使得HA大量积累.     

Characterization of humic acids extracted from sewage sludge-amended oxisols by electron paramagnetic resonance

In tropical soils, the high turnover rate and mineralization of organic matter (OM) associated with intensive agricultural use, generally leads to faster soil degradation than that observed in temperate climatic zones. The application of sewage sludge to the soils is one proposed method of maintaining soil organic matter, and is also an alternative method of disposing of this waste product. As well as containing large quantities of OM, sludge is also a significant source of supplementary nitrogen, phosphorus and other essential nutrients for plant growth. However, it is necessary to understand the qualitative and quantitative changes that take place in the OM in soil treated with sewage sludge. The approach of the present study was intended to identify possible structural changes caused by sewage sludge applications on soil humic acids (HAs). The HAs extracted from a Typic Achrortox under sewage sludge applications were characterized by electron paramagnetic resonance (EPR) spectroscopy. The soil samples were collected from a field experiment designed to evaluate the effects of different doses of sewage sludge on corn growth and development in Brazil. The sewage sludge originated from urban waste treated at the sewage sludge treatment station in the city of Franca, state of Sao Paulo, Brazil. The following soil treatments were studied: control (non-cultivated soil under natural vegetation (NC)), control soil amended with NPK (conventional corn fertilization) and four treatments N1, N2, N4 and N8 with applications of 3.5, 7, 14 and 28 Mg ha⁻¹ of sewage sludge (dry matter), respectively. HAs were extracted from the surface layer using the methodology of the International Humic Substance Society (IHSS). Fe³⁺ and VO²⁺ ions complexed with HAs, and also semiquinone-type free radical (SFR) at concentrations of approximately 2.0 × 10¹⁸ spins g⁻¹ HA were identified in EPR spectra. The levels of SFR were lower for treatments where the applied sewage sludge doses were equivalent to four and eight times the normal doses of N mineral fertilization, reaching values of 1.7 × 10¹⁸ and 1.24 × 10¹⁸ spins g⁻¹ HA, respectively. The observed decrease in SFR content as sewage sludge dose was increased, was probably associated with the incorporation of less aromatic components into HAs originating from the sewage sludge.     

Phospholipid fatty acid composition of a 2,4,6-trinitrotolune contaminated soil and an uncontaminated soil as affected by a humification remediation process

Analysis of phospholipid fatty acids (PLFAs) was performed to investigate effects of 2,4,6-trinitrotoluene (TNT) contamination and soil remediation on microbial biomass and community structure. A TNT-contaminated and an uncontaminated soil from a former ammunition plant were analysed before and after a humification/remediation process. TNT contamination reduced microbial biomass but indicated only minor differences in PLFA composition between the contaminated and uncontaminated soils. The humification process increased microbial biomass and altered soil PLFA patterns to a larger degree than did TNT contamination.     

Mineralisation and humification of plant matter in soil samples as a tool in the testing of soil quality

Since life is inevitably dependent on the assimilation and dissimilation of carbon, and since most of the organic carbon is bound in soil humic matter, the mineralisation and humification of plant carbon in soil should be monitored, so as to evaluate soil quality and avoid ecological risks. For this reason we suggested an incubation test of thirty days in duration with soil samples amended or not with lucerne meal as a source of plant carbon. During the incubation period, CO₂ release is to be measured repeatedly, and thereafter contents of humic acids (HA) and fulvic acids (FA) should be estimated and compared with those determined in the original soil samples. Our results obtained with samples from non-fertilized or long-term (?>?40 years) fertilized plots of two field trials indicate that fertilization by NPK?+ farm yard manure resulted in (i) a slight enhancement of C mineralisation, i.e., CO₂ release from soil, and (ii) a significant increases of HA and FA contents. Soil samples from a reclaimed mine spoil, and either fertilised or not with sewage sludge did not show distinct differences in the same parameters. The practicability of the incubation test as a tool in the monitoring of soil quality should be further proved using samples from differently affected soils.     

Evaluation of soil quality parameters in a tropical paddy soil amended with rice residues and tree litters

Laboratory and greenhouse experiments were conducted to study the effects of applications of rice residue and Pongamia pinnata and Azadirachta indica leaf litters on biochemical properties (extraction yield of humus, composition of humus, microbial biomass carbon, activities of urease and acid phosphatase) of a lowland rice soil under flooded conditions. Bulk soil sample collected from the Mandya paddy fields was used for the green house trials and the laboratory incubation studies. The organic materials were added at three rates – zero, 25.0 g carbon kg⁻¹ (2.5% C) and 50.0 g carbon kg⁻¹ dry soil (5.0% C). Results showed that tree leaf litter and rice residue at 5.0% C rate decreased instantaneous decay constant (k), there by retarded the rate of C mineralization. Carbon contents of HA increased with the rate of C added. Study of delta–log K values and C contents of humic acids revealed that greatest molecular weight of HA was in the pongamia litter treatment, followed by neem litter and rice residue. Grain and straw yields of rice crop in the pot culture study were statistically correlated to the soil quality parameters. Neem and pongamia tree litter incorporation at 2.5% C could be considered for improving soil health and crop yields of rice under flooded conditions; however, application at higher rates significantly (P ≤ 0.05) lowered total dry matter production in rice, despite favorable soil health parameters such as humic yields, microbial biomass – C content and acid phosphatase and urease activity. Among different soil health parameters, microbial quotient was found to be more sensitive indicator of decline in soil quality.     

麦秆湿解固体产物腐殖化与炭化特性分析

为了研究生物质湿解过程中固体产物的腐殖化和炭化特性,以麦秆为原料,在高温高压反应釜中,进行反应温度为220℃、停留时间30~180 min条件下的湿解实验研究,结合X射线衍射(XRD)和傅里叶红外光谱(FTIR)检测分析结果对麦秆湿解固体产物的微观结构和化学组成进行了深入分析。研究发现,麦秆经过湿解处理,固体产物中C元素质量分数从停留时间30 min时的45.86%逐渐增加至180 min时的49.06%;在停留时间为60 min时,XRD谱图上在衍射角2θ约为26°附近就已经出现强峰,微晶结构接近于石墨,并随停留时间的增加,芳香化和炭化程度逐渐提高;固体产物中含有丰富的腐殖质,具有大量的芳香结构和含氧官能团。