太湖流域典型稻麦轮作农田稻季不施磷的农学及环境效应探究

以太湖流域典型稻麦轮作农田为研究对象开展大田试验,通过设置稻麦季均施磷（PR+W,当前农民施肥习惯）、麦季施磷稻季不施磷（PW）、稻季施磷麦季不施磷（PR）以及稻麦季均不施磷（Pzero,对照）四种施磷处理,提出麦季施磷稻季不施磷（PW）的减磷措施。四年八季作物研究结果表明：与传统PR+W处理相比较,PW处理的作物籽粒和秸秆产量均无显著变化,但却提高磷肥利用率3.54%,同时降低土壤速效磷累积量10.5%~36.7%,减少径流总磷浓度12.0%。据此推算,如果一个稻季不施磷肥,太湖流域1.02×106 hm2水稻土四年可节约P2O5约24万t,估算该流域每年可直接节约肥料投入成本3.06亿元。这表明采取麦季施磷稻季不施磷的减磷措施不仅能够保持稳产,而且可节约磷肥（磷矿）资源,降低水环境污染风险,具有农学、环境以及经济效益的三赢潜力。     

Effect of P fertilizer reduction regime on soil Olsen-P, root Fe-plaque P, and rice P uptake in rice-wheat rotation paddy fields

In agricultural systems, it is vital to use limited yet optimal phosphorus(P) resources, because excessive P fertilizer application leads to the accumulation of P in soil, increasing the risk of environmental pollution and causing the waste and exhaustion of P resources. In a rice-wheat rotation system, omitting P fertilizer application in the rice-growing season is a good alternative;however, how this P fertilization reduction influences changes in P in the soil-root-aboveground system is unclear. In this study, after a seven-year rice-wheat rotation at the Yixing(YX) and Changshu(CS) sampling sites, China, compared with P fertilization in rice-and wheat-growing seasons(PR+W), reduced P fertilization(no P fertilizer application in either season, P0;P fertilization only in wheat-growing seasons, PW;and P fertilization only in rice-growing seasons, PR) did not result in substantial variation in crop biomass. The PW treatment did not reduce crop total P, root iron(Fe)-plaque P, and soil Olsen-P at three stages of rice growth(seedling, booting, and harvesting stages) at the YX and CS sites. In contrast, concentrations of soil Olsen-P, aboveground crop total P, and root Fe-plaque P decreased in the P0 treatment by 45.8%–81.0%,24.6%–30.9%, and 45.6%–73.4%, respectively. In addition, a significant negative correlation was observed between the root Fe-plaque P and crop biomass at the two sites. Significant positive correlations were also observed between root Fe-plaque P and root total P, crop total P, and soil Olsen-P. In addition, the results of a redundancy analysis revealed that soil alkaline phosphatase(ALP) played a major role in the supply of P in soil, and was closely associated with root Fe-plaque P. The results of this study will enhance the understanding of the changes in P in the soil-root-aboveground system, particularly under P fertilizer reduction regimes.     

Ecosystem partitioning of N-glycine after long-term climate and nutrient manipulations, plant clipping and addition of labile carbon in a subarctic heath tundra

Low temperatures and high soil moisture restrict cycling of organic matter in arctic soils, but also substrate quality, i.e. labile carbon (C) availability, exerts control on microbial activity. Plant exudation of labile C may facilitate microbial growth and enhance microbial immobilization of nitrogen (N). Here, we studied ¹⁵N label incorporation into microbes, plants and soil N pools after both long-term (12 years) climate manipulation and nutrient addition, plant clipping and a pulse-addition of labile C to the soil, in order to gain information on interactions among soil N and C pools, microorganisms and plants. There were few effects of long-term warming and fertilization on soil and plant pools. However, fertilization increased soil and plant N pools and increased pool dilution of the added ¹⁵N label. In all treatments, microbes immobilized a major part of the added ¹⁵N shortly after label addition. However, plants exerted control on the soil inorganic N concentrations and recovery of total dissolved ¹⁵N (TD¹⁵N), and likewise the microbes reduced these soil pools, but only when fed with labile C. Soil microbes in clipped plots were primarily C limited, and the findings of reduced N availability, both in the presence of plants and with the combined treatment of plant clipping and addition of sugar, suggest that the plant control of soil N pools was not solely due to plant uptake of soil N, but also partially caused by plants feeding labile C to the soil microbes, which enhanced their immobilization power. Hence, the cycling of N in subarctic heath tundra is strongly influenced by alternating release and immobilization by microorganisms, which on the other hand seems to be less affected by long-term warming than by addition or removal of sources of labile C.     

Nutrient management effects on organic carbon pools in a sandy loam soil under rice-wheat cropping

Agricultural management practices are known to influence soil organic C. While changes in total organic C (TOC) are relatively less discernible over short to medium-term, some extractable pools of TOC are considered early indicators of changes in TOC. Therefore, to devise nutrient management practices that can lead to C sequestration, it is important to study their effect on soil organic C pools that may respond rapidly to management. We studied the impact of balanced (NPK) and imbalanced (N, NP, NK and PK) application of fertilizer nutrients without and with farmyard manure (FYM) on total and labile pools of organic C viz. water soluble (WEOC), potassium permanganate oxidizable (KMnO₄-C), microbial biomass (MBC) and fractions of decreasing oxidizability after 5-cycles of rice-wheat cropping. Integrated use of NPK and FYM significantly increased TOC and extractable C pools in both surface (0–7.5 cm) and sub-surface (7.5–15 cm) soil. Majority of TOC (72%) was stabilized in less labile and recalcitrant fractions; the magnitude being higher under balanced (NPK+FYM) than imbalanced nutrient management (N+FYM). The results showed that balanced fertilizer application conjointly with FYM besides enlarging TOC pool favorably impacts soil organic matter composition under rice-wheat system.     

Significance of temperature and water availability for soil phosphorus transformation and microbial community composition as affected by fertilizer sources

Little is known about the effects of temperature and drying–rewetting on soil phosphorus (P) fractions and microbial community composition in regard to different fertilizer sources. Soil P dynamics and microbial community properties were evaluated in a soil not fertilized or fertilized with KH₂PO₄ or swine manure at two temperatures (10 and 25 °C) and two soil water regimes (continuously moist and drying–rewetting cycles) in laboratory microcosm assays. The P source was the dominant factor determining the sizes of labile P fractions and microbial community properties. Manure fertilization increased the content of labile P, microbial biomass, alkaline phosphomonoesterase activity, and fatty acid contents, whereas KH₂PO₄ fertilization increased the content of labile inorganic P and microbial P. Water regimes, second to fertilization in importance, affected more labile P pools, microbial biomass, alkaline phosphomonoesterase activity, and fatty acid contents than temperature. Drying–rewetting cycles increased labile P pools, decreased microbial biomass and alkaline phosphomonoesterase activity, and shaped the composition of microbial communities towards those with greater percentages of unsaturated fatty acids, particularly at 25 °C in manure-fertilized soils. Microbial C and P dynamics responded differentially to drying–rewetting cycles in manure-fertilized soils but not in KH₂PO₄-fertilized soils, suggesting their decoupling because of P sources and water regimes. Phosphorus sources, temperature, and water regimes interactively affected the labile organic P pool in the middle of incubation. Overall, P sources and water availability had greater effects on P dynamics and microbial community properties than temperature.     

Soil carbon pools under long-term rice-wheat cropping system in Inceptisols of Indian Himalayas

Knowledge about soil organic carbon (SOC) stock and its allocation into different pools is important for global food and environmental security. Accordingly, an attempt is made in the present study to investigate into the dynamics of SOC pools i.e. total soil organic carbon (TOC), oxidisable organic carbon (OC) and its different fractions viz. very labile (C_VL), labile (C_L), less labile (C_LL) and non-labile (C_NL) in soils under a 26 years old long-term experiment with rice (Oryza sativa L) – wheat (Triticum aestivum L) cropping system on Inceptisols under humid agro-climatic region of India with different soil management practices (control, 100% recommended dose of NPK, and 50% recommended dose of NPK + 50% N through farmyard manure (FYM). Of the several pools analyzed, a higher proportion of C was found in labile pool followed by very labile, non-labile, and less labile ones constituting about 46, 26.5, 20 and 7.3% of the total organic C at surface soil. The NPK+FYM treatment was found to have higher SOC pools, lability index (LI), recalcitrance indices and stratification ratio as compared to others. Results indicated that balanced fertilization with inorganic and organics is important for maintaining overall sustainability of the rice-wheat system.     

The use of biologically based phosphorus fractions to evaluate soil P availability in reduced P‐input paddy soils

Chemical soil phosphorus (P) extraction has been widely used to characterize and understand changes in soil P fractions; however, it does not adequately capture rhizosphere processes. In this study, we used the biologically based phosphorus (BBP ) grading method to evaluate the availability and influencing factors of soil P under four P fertilizer regimes in a typical rice–wheat cropping rotation paddy field. Soil P was assessed after seven rice‐growth seasons at multiple growth stages: the seedling, the booting and the harvest stage. Soil CaCl₂‐P, citrate‐P and HC l‐P (inorganic P, Pi) as well as enzyme‐P (organic P, Po) were not significantly different between soil treated with P fertilizer during the wheat season only (PW ) and during the rice season only (PR ) compared with soil treated during both the rice and the wheat seasons (PR +W) at all three rice‐growth stages. No P fertilizer application during either season (Pzero) significantly reduced the concentration of soil citrate‐P and HC l‐P at the rice‐seedling and harvest stages. Significant correlations were observed between the HC l extraction and Olsen‐P (R ² = 0.823, P  <  0.001), followed by enzyme‐P (R ² = 0.712, P  <  0.001), citrate‐P (R ² = 0.591, P  <  0.001) and CaCl₂‐P (R ² = 0.133, P  <  0.05). Further redundancy analysis (RDA ) suggested that soil alkaline phosphatase (S‐ALP ) activity played a role in soil P speciation changes and was significantly correlated with enzyme‐P, citrate‐P and HC l‐P. These results may improve our ability to characterize and understand changes in soil P status while minimizing the overapplication of P fertilizer.     

水旱轮作与林地利用方式对中国滨海滩涂土壤微生物生物量碳的影响

Rice-wheat rotation and poplar afforestation are two typical land use types in the coastal reclaimed flatlands of eastern China.This study investigated two rice-wheat rotation lands(one reclaimed from 1995 to 2004 and cultivated since 2005, RW1, and the other reclaimed from 1975 to 1995 and cultivated since 1996, RW2) and a poplar woodland(reclaimed from 1995 to 2004 and planted in2004, PW1) to determine the effects of land use types and years of cultivation on soil microbial biomass and mineralizable carbon(C) in this coastal salt-affected region. The results showed that the soil in PW1 remained highly salinized, whereas desalinization was observed in RW1. The total organic C(TOC) in the top soil of PW1 and RW1 did not show significant differences, whereas at a soil depth of 20–30 cm, the TOC of RW1 was approximately 40%–67% higher than that of PW1. The TOC of 0–30-cm soil in RW2 was approximately 37% higher than that in RW1. Microbial biomass C(MBC) and mineralizable C(MNC) exhibited the trend of RW2 RW1 PW1. Sufficient nutrition with more abundant C substrates resulted in higher MBC and MNC, and soil respiration rates were negatively correlated with C/N in RW1 and RW2. Nutrient deficiency and high salinity played key roles in limiting MBC in PW1. These suggested that rice-wheat rotation was more beneficial than poplar afforestation for C accumulation and microbial biomass growth in the coastal salt-affected soils.     

秸秆还田对稻麦轮作农田活性有机碳组分含量、酶活性及产量的短期效应

【目的】秸秆还田作为一种有效的秸秆处理方式不仅能够提高土壤肥力,增加作物产量,还可以缓解农田生态压力。研究稻麦轮作系统下不同秸秆还田量对土壤活性有机碳库、酶活性和作物产量的短期影响,可为提出适宜当地生产的秸秆还田量提供理论依据。【方法】利用稻麦轮作农田定位试验进行了研究。采用随机区组设计,设7个处理,以稻麦季秸秆均不还田为对照处理(CK),6个不同秸秆还田量处理。测定了秸秆还田后土壤活性有机碳库和土壤酶活性的变化,稻麦产量以及三者之间的相关关系。【结果】1)与秸秆不还田处理相比,试验范围内的秸秆还田量能在一定程度上提高土壤活性碳组分的含量和土壤酶活性,并能增加水稻和小麦的产量及其构成因素;2)土壤总有机碳和微生物生物量碳的含量随着秸秆还田量增加,增幅呈先增大后减小的趋势,以连续两季50%秸秆还田量处理下显著较高,而水溶性有机碳、活性有机碳含量和碳库管理指数在连续两季25%秸秆还田量处理下最高;3)相比秸秆不还田处理,连续两季25%秸秆还田量对土壤脲酶、过氧化氢酶和蔗糖酶活性的影响均最显著;4)水稻和小麦的产量均为在连续两季25%和50%秸秆还田量处理下增产较显著,与秸秆不还田相比,水稻增产达9.0%,小麦增产达11.45%;5)土壤碳库、土壤酶活性以及水稻和小麦产量之间均存在显著相关。【结论】在本试验条件下,连续两季25%和50%秸秆还田量表现出显著提高土壤碳汇能力和增加作物产量的优势。     

Soil organic matter content and chemical composition under two rotation management systems in a Mediterranean climate

The crop rotation system in organic farming is a determinant factor to accumulate and preserve soil organic matter (SOM), and in depth knowledge on its effects is still lacking. Tillage intensity in particular is crucial to maintain soil aggregates and protect SOM from degradation. The evolution of SOM was tested in two adjacent fields under two different rotation cropping systems (low-intensity tillage and high-intensity tillage), and the effect of a further cultivation of legume in both fields was evaluated using ¹³carbon (C)-nuclear magnetic resonance (NMR) and elemental analysis of samples isolated through combined aggregate size and density fractionation. The two adjacent fields had been managed using the following organic farming methods for 13 seasons since 1998: i) alfalfa-based, with nitrogen (N) enrichment and low-frequency tillage with alfalfa (Medicago sativa) (9 seasons), winter wheat (Triticum durum) (3 seasons), and broad bean (Vicia faba) (1 season) and ii) cereal-based, with N depletion and annual tillage with barley (Hordeum vulgare) (7 seasons), sunflower (Helianthus annuus) (2 seasons), broad bean (Vicia faba) (3 seasons), and bare fallow (1 season). Soil sampling was carried out at the end of the 13-year rotation (T0, November 2011) and after winter wheat and chickpea cultivation in both fields over two subsequent years (T1, July 2013). Bulk organic C was significantly higher in the alfalfa-based system than in the cereal-based system at both T0 and T1, with SOM occluded in soil aggregates and associated with mineral particles. In terms of the macroaggregates heavy fraction at T0, the alfalfa-based field contained twice the organic C of that in the cereal-based field, as well as three times the organic C in the occluded particulate organic matter (POM). The occluded POM (oPOM) had a lower aryl/O-alkyl C ratio in the alfalfa-based system than in the cereal-based system, suggesting that oPOM undergoes a lower degree of decomposition during low-intensity management. The aryl/O-alkyl C ratios of the macro-and microaggregate oPOM decreased from T0 to T1 in the cereal-based system, suggesting increased protection of these fractions by soil aggregates. Thus, including legumes in crop rotation appears to positively affect the accumulation of SOM associated with mineral particles and within soil aggregates.     

Growth and rhizosphere P pools of legume–wheat rotations at low P supply

Legume pre-crops may increase P uptake of the following wheat, but the mechanisms behind this effect are unclear. A rotation study was carried out to assess the concentrations of rhizosphere P pools of three grain legumes and wheat (phase 1) and their effects on P uptake and P pools in the rhizosphere of the following wheat (phase 2). Faba bean, chickpea, white lupin and wheat were grown for 10 weeks in a loamy sand soil with low P availability. The following wheat was grown in the pre-crop soil with and without addition of pre-crop residues. Among the pre-crops, white lupin had the strongest effect on the P pools; it depleted the labile P pools, resin P and NaHCO₃-Pi and also the less labile P pools, NaOH-Pi and residual P; whereas the concentration of NaHCO₃-Po was higher than that in the rhizosphere of the other pre-crops. White lupin had a smaller biomass compared to faba bean which depleted the P pools to a lesser extent. Phosphorus uptake of the following wheat was greatest in white lupin pre-crop soil. Chickpea increased P uptake of the following wheat when residues were added. In the presence of residues, wheat after legumes depleted labile P pools to a greater extent than wheat after wheat, but this coincided with greater P uptake only in wheat after chickpea and white lupin, which may be explained by the small root biomass of wheat after faba bean. The results show that the greater P uptake of the following wheat induced by pre-crops may be due to two mechanisms: P mobilisation (white lupin) or P addition with legume residues (chickpea). This study further showed that P uptake by a crop is only partly a function of the depletion of P in the rhizosphere; another important factor is the ability to exploit a large soil volume.     

Seasonal fluctuations in soil microbial biomass carbon,phosphorus, and activity in no-till and reduced-chemical-input maize agroecosystems

Summary The soil microbial biomass contains important labile pools of C, N, P, and S, and fluctuations in its size and activity can significantly influence crop productivity. In cropping systems where fertilizer use is reduced or eliminated and green-manure legumes are used, nutrient availability is more directly linked to C-cycle dynamics. We observed the fluctuations in microbial biomass C and P, and in microbial biomass activity over three cropping seasons in continuous maize and 2-year maize-wheat-soybean rotation agroecosystems under no-till and reduced-chemical-input management. We estimated the concentrations of microbial C and P using fumigation-incubation and fumigation-extraction techniques for the surface 20 cm of Cecil and Appling series soils (clayey, kaolinitic, thermic, Typic Kanhapludults). There were significant seasonal fluctuations in microbial C and P under all cropping systems. Generally, the magnitude of fluxes and the quantity of microbial C and P tended to be higher in reduced-chemical-input systems due to tillage and incorporation of crop, weed, and legume residues. Over 3 years, the means for microbial C were 435 under reduced-input maize; 289 under no-till maize; 374 und the reduced-input crop rotation; and 288 mg kg^-1 soil under the no-till rotation. The means for microbial P were 5.2 under reduced-input maize; 3.5 under no-till maize; 5.0 under the reduced-input rotation; and 3.5 mg kg^-1 soil under the no-till rotation. Estimates of microbial activity, derived from CO₂–C evolution and specific respiratory activity (mg CO₂–C per mg biomass C), suggest that reduced-chemical-input management may cause a larger fraction of the biomass to be relatively inactive but may also increase the activity of the remaining fraction over that in no-till. Thus in these specific systems, the turnover of C and P through the microbial biomass with a reduced chemical input to the soil may be higher than under a no-till system.     

Microbial biomass in soils under different tillage and crop rotation systems

A long-term study on the effect of different crop rotations [soybean/wheat, S/W; maize/wheat, M/W or cotton/wheat, C/W] and tillage regimes [no-tillage (NT) or conventional tillage (CT)] on microbial biomass and other soil properties is reported. The experiment was established in 1976 in southern Brazil as a split-plot experimental design in three replications. Soil samples were taken in 1997 and 1998 at 0- to 5-, 5- to 10- and 10- to 20-cm depths and evaluated for microbial biomass C, N, P and S by direct extraction methods. The NT system showed increases of 103%, 54%, 36%, and 44% for microbial biomass C, N, P, and C_mic:C_org percentage, respectively at the 0- to 5-cm depth. NT systems also increased the C to N:S:P ratios. These results provide evidence that tillage or crop rotation affect microbial immobilization of soil nutrients. The larger amount of C immobilized in microbial biomass suggests that soil organic matter under NT systems provides higher levels of more labile C than CT systems.     

Soil organic phosphorus and microbial community composition as affected by 26 years of different management strategies

Agricultural management can affect soil organic matter chemistry and microbial community structure, but the relationship between the two is not well understood. We investigated the effect of crop rotation, tillage and stubble management on forms of soil phosphorus (P) as determined by solution ³¹P nuclear magnetic resonance spectroscopy and microbial community composition using fatty acid methyl ester analysis in a long-term field experiment (26 years) on a Chromic Luvisol in New South Wales, Australia. An increase in soil organic carbon, nitrogen and phosphorus compared to the beginning of the experiment was found in a rotation of wheat and subterranean clover with direct drill and mulching, while stubble burning in wheat–lupin and wheat–wheat rotations led to soil organic matter losses. Microbial biomass was highest in the treatment with maximum organic matter contents. The same soil P forms were detected in all samples, but in different amounts. Changes in organic P occurred mainly in the monoester region, with an increase or decrease in peaks that were present also in the sample taken before the beginning of the experiment in 1979. The microbial community composition differed between the five treatments and was affected primarily by crop rotations and to a lesser degree by tillage. A linkage between soil P forms and signature fatty acids was tentatively established, but needs to be verified in further studies.     

Using diffusive gradients in thin films technique for in-situ measurement of labile phosphorus around Oryza sativa L. roots in flooded paddy soils

Behavior of phosphorus(P) in flooded rice soil is controlled by iron(Fe) redox cycling in root-zone. In this study, we applied a novel approach—the diffusive gradients in thin films(DGT) technique—for investigating the in-situ distribution of labile phosphorus(P) and Fe in close proximity to Asian rice(Oryza sativa L.) roots at submillimeter to millimeter spatial resolutions during the seedling and booting stages. We conducted a seven-year field experiment under rice-wheat rotation with different P fertilizer treatments. The results showed a significant and strong positive relationship of the average DGT-labile P concentration with soil Olsen P(R2= 0.77, P < 0.01) and with rice total P concentration(R²= 0.62, P < 0.05). Furthermore, results on one-and two-dimensional changes of DGT-labile P indicated that fertilization only in the wheat season produced sufficient amounts of labile P in the flooded paddy soils, similar to when fertilizer was applied only in the rice season;dissolved P concentrations, however, were lower. A co-occurrence and significant positive correlation(P < 0.01) between DGT-labile P and Fe indicated Fe-coupled mobilization of P in flooded paddy soils. These results collectively indicated that the DGT technique provided information on in-situ distribution of labile P and its variability in close proximity to rice roots. This suggests that the DGT technique can improve our understanding of in-situ and high-resolution labile P processes in paddy soils and can provide useful information for optimizing P fertilization.     

Short-term decomposition of <Superscript>14</Superscript>C-labelled glucose in a fluvo-aquic soil as affected by lanthanum amendment

In this study, we investigated the effects of lanthanum (La), one of the rare earth elements (REEs), on microbial biomass C as well as the decomposition of ¹⁴C-labelled glucose in a fluvo-aquic soil in 28 days. The soil was collected from the field plots under maize/wheat rotation in Fengqiu Ecological Experimental Station of Chinese Academy of Sciences, Henan Province, China. Application of La decreased soil microbial biomass C during the experimental period, and there was a negative correlation (P < 0.01) between microbial biomass and application rate of La. La increased microbial biomass ¹⁴C after ¹⁴C glucose addition, and the increase was significant (P < 0.05) at the rates of more than 160 mg kg⁻¹ soil. La slightly increased ¹⁴CO₂ evolution at lower rates of application but decreased it at higher rates 1 day after ¹⁴C glucose addition, while there was no significant effect from days 2 to 28. For the cumulative ¹⁴CO₂ evolution during the incubation of 28 days, La slightly increased it at the rates of less than 120 mg kg⁻¹ soil, while significantly decreased (P < 0.05) it at the rate of 200 mg kg⁻¹ soil. The results indicated that agricultural use of REEs such as La in soil could decrease the amount of soil microbial biomass and change the pattern of microbial utilization on glucose C source in a short period.     

长期水旱轮作条件下紫色土养分供应能力的研究

通过10年稻—麦水旱轮作定位试验,研究了紫色土生产力以及N、P、K自然供应能力和变化。结果表明,在水旱轮作下,紫色土对稻、麦产量的地力贡献率平均为58%和51%,肥料的增产贡献率可达42%～49%。紫色土N、P、K养分自然供给力在小麦上分别为60%、70%、91%,在水稻上分别为70%、90%、92%,小麦对肥料的依赖性高于水稻;N、P、K养分的自然供应量在小麦上分别为37.1、5.4、45.9kg.hm2,在水稻上分别为81.8、13.6、103.0kg.hm2。随着试验年份的延长,旱作季节紫色土N、P、K养分的供应能力逐年降低,水作季节N、P、K供应力则相对稳定,旱作对地力的消耗比水作大。环境输入的养分在维持水田稳定供肥能力发挥了重要作用。紫色土这种基础养分供给力可维持每年生产小麦1.4t.hm2、稻谷3.5t.hm2左右。     

Turnover of soil organic matter and of microbial biomass under C3-C4 vegetation change: Consideration of C fractionation and preferential substrate utilization

Two processes contribute to changes of the δ¹³C signature in soil pools: ¹³C fractionation per se and preferential microbial utilization of various substrates with different δ¹³C signature. These two processes were disentangled by simultaneously tracking δ¹³C in three pools - soil organic matter (SOM), microbial biomass, dissolved organic carbon (DOC) - and in CO₂ efflux during incubation of 1) soil after C₃-C₄ vegetation change, and 2) the reference C₃ soil.The study was done on the Ap horizon of a loamy Gleyic Cambisol developed under C₃ vegetation. Miscanthus giganteus - a perennial C₄ plant - was grown for 12 years, and the δ¹³C signature was used to distinguish between ‘old’ SOM (>12 years) and ‘recent’ Miscanthus-derived C (<12 years). The differences in δ¹³C signature of the three C pools and of CO₂ in the reference C₃ soil were less than 1‰, and only δ¹³C of microbial biomass was significantly different compared to other pools. Nontheless, the neglecting of isotopic fractionation can cause up to 10% of errors in calculations. In contrast to the reference soil, the δ¹³C of all pools in the soil after C₃-C₄ vegetation change was significantly different. Old C contributed only 20% to the microbial biomass but 60% to CO₂. This indicates that most of the old C was decomposed by microorganisms catabolically, without being utilized for growth. Based on δ¹³C changes in DOC, CO₂ and microbial biomass during 54 days of incubation in Miscanthus and reference soils, we concluded that the main process contributing to changes of the δ¹³C signature in soil pools was preferential utilization of recent versus old C (causing an up to 9.1‰ shift in δ¹³C values) and not ¹³C fractionation per se.Based on the δ¹³C changes in SOM, we showed that the estimated turnover time of old SOM increased by two years per year in 9 years after the vegetation change. The relative increase in the turnover rate of recent microbial C was 3 times faster than that of old C indicating preferential utilization of available recent C versus the old C.Combining long-term field observations with soil incubation reveals that the turnover time of C in microbial biomass was 200 times faster than in total SOM. Our study clearly showed that estimating the residence time of easily degradable microbial compounds and biomarkers should be done at time scales reflecting microbial turnover times (days) and not those of bulk SOM turnover (years and decades). This is necessary because the absence of C reutilization is a prerequisite for correct estimation of SOM turnover. We conclude that comparing the δ¹³C signature of linked pools helps calculate the relative turnover of old and recent pools.     

Gaseous carbon emission in relation to soil carbon fractions and microbial diversities as affected by organic amendments in tropical rice soil

The interactive effects of moisture and organic amendments (farmyard manure (FYM), crop residue (CR) and green manure (GM) (Sesbania aculeata) on gaseous carbon (C) emission, soil labile C fractions, enzymatic activities and microbial diversity in tropical, flooded rice soil were investigated. The amendments were applied on equal C basis in two moisture regimes, that is, aerobic and submergence conditions. The CO₂ production was significantly higher by 22% in aerobic than in submergence condition; on the contrary, the CH₄ production was 27% higher under submergence condition. The labile C fractions were significantly higher in GM by 26% under aerobic and 30% under submergence conditions, respectively, than control (without any kind of fertilizer or amendments). Eubacterial diversity identified by PCR-DGGE method (polymerase chain reaction coupled with denaturant gradient gel electrophoresis) was higher under GM followed by FYM, CR, and control and it is pronounced in submerged condition. GM favored the labile C accumulation and biological activities under both submergence and aerobic conditions, which makes it most active for soil–plant interactions compared to other organic amendments. Considering environmental sustainability, the use of GM is the better adoptable option, which could enhance labile C pools, microbial diversities in soil and keep soil biologically more active.     

Tree girdling provides insight on the role of labile carbon in nitrogen partitioning between soil microorganisms and adult European beech

Nitrogen (N) cycling in terrestrial ecosystems is complex since it involves the closely interwoven processes of both N uptake by plants and microbial turnover of a variety of N metabolites. Major interactions between plants and microorganisms involve competition for the same N species, provision of plant nutrients by microorganisms and labile carbon (C) supply to microorganisms by plants via root exudation. Despite these close links between microbial N metabolism and plant N uptake, only a few studies have tried to overcome isolated views of plant N acquisition or microbial N fluxes. In this study we studied competitive patterns of N fluxes in a mountainous beech forest ecosystem between both plants and microorganisms by reducing rhizodeposition by tree girdling. Besides labile C and N pools in soil, we investigated total microbial biomass in soil, microbial N turnover (N mineralization, nitrification, denitrification, microbial immobilization) as well as microbial community structure using denitrifiers and mycorrhizal fungi as model organisms for important functional groups. Furthermore, plant uptake of organic and inorganic N and N metabolite profiles in roots were determined.Surprisingly plants preferred organic N over inorganic N and nitrate (NO₃⁻) over ammonium (NH₄⁺) in all treatments. Microbial N turnover and microbial biomass were in general negatively correlated to plant N acquisition and plant N pools, thus indicating strong competition for N between plants and free living microorganisms. The abundance of the dominant mycorrhizal fungi Cenococcum geophilum was negatively correlated to total soil microbial biomass but positively correlated to glutamine uptake by beech and amino acid concentration in fine roots indicating a significant role of this mycorrhizal fungus in the acquisition of organic N by beech. Tree girdling in general resulted in a decrease of dissolved organic carbon and total microbial biomass in soil while the abundance of C. geophilum remained unaffected, and N uptake by plants was increased. Overall, the girdling-induced decline of rhizodeposition altered the competitive balance of N partitioning in favour of beech and its most abundant mycorrhizal symbiont and at the expense of heterotrophic N turnover by free living microorganisms in soil. Similar to tree girdling, drought periods followed by intensive drying/rewetting events seemed to have favoured N acquisition by plants at the expense of free living microorganisms.