Impacts of wetting-drying cycles on short-term carbon and nitrogen dynamics in Amynthas earthworm casts

Effects of earthworm casts on soil nutrient dynamics and their responses to changing moisture availability in subtropical ecosystems remain poorly understood. This study aimed to examine short-term carbon(C) and nitrogen(N) dynamics and their interactions with wetting-drying cycles in three different structural forms(i.e., granular, globular, and heap-like) of Amynthas earthworm casts. The rates of C and N mineralization in the earthworm casts were examined under two different wetting-drying cycles(i.e., 2-d and 4-d wetting intervals) using a rainfall simulation experiment. After three simulated rainfall events, subsamples of the earthworm casts were further incubated for 4 d for the determination of CO_2 and N_2O fluxes. The results of this study indicated that the impacts of wetting-drying cycles on the short-term C and N dynamics were highly variable among the three cast forms, but wetting-drying cycles significantly reduced the cumulative CO_2 and N_2O fluxes by 62%–83% and 57%–85%, respectively, when compared to the control without being subjected to any rainfall events. The C mineralization rates in different cast forms were affected by the amount of organic substrates and N content in casts, which were associated with the food preference and selection of earthworms. Meanwhile, the cumulative N_2O fluxes did not differ among the three cast forms. Repeated wetting and drying of casts not only enhanced aggregate stability by promoting bonds between the cast particles, but also inhibited microbial survival and growth during the prolonged drying period, which together hindered decomposition and denitrification. Our findings demonstrated that the interactions between the structural forms, aggregate dynamics, and C and N cycling in the earthworm casts were highly complex.     

秸秆还田对麦粱两熟农田土壤团聚体特征的短期效应

冬小麦—夏高粱种植系统作为一种新型农业两熟制系统,是山西省杂粮可持续发展的一项有效措施。为阐明该种植系统农田土壤团聚体粒级分布及稳定性对秸秆还田量的短期响应,试验基于麦粱种植系统,分析了不还田（CK）、半量还田（HR）和全量还田（WR）对土壤团聚体粒级分布特征和稳定指数的影响。结果表明:秸秆还田后,能够显著降低0—30 cm土层 > 10 mm和 < 0.25 mm粒级机械稳定性团聚体含量,增加0.25～2 mm各亚粒级水稳性大团聚体含量,同时显著降低了土壤团聚体破坏率和不稳定团粒指数（p < 0.05）;全量秸秆还田后较半量秸秆还田对农田土壤团聚体特征改善效果更为明显,但对10—20,20—30 cm土层改善效果逐渐减弱;全量还田相比半量还田,土壤机械稳定性团聚体平均重量直径、几何平均直径和大团聚体（> 0.25 mm）含量分别显著降低了12.2%,23.0%和5.3%,并显著提升了水稳性团聚体几何平均直径和大团聚体（> 0.25 mm）含量,降低了水稳性团聚体分形维数（p < 0.05）。此外,土壤团聚体稳定性与有机碳含量、孔隙度、含水量和作物产量呈显著正相关（p < 0.05）。综合表明,全量还田在短期年限内能够显著提高土壤团聚体稳定性,是改善晋中区麦粱两熟农田土壤团粒结构和增加作物产量的有效措施。     

尿素氮形态转化对腐殖酸的响应

通过模拟试验研究了不同用量腐殖酸对土壤中尿素氮形态转化的影响。结果表明,腐殖酸对尿素态氮形态转化的影响受其施用量的制约。与对照相比,低浓度腐殖酸(<15gkg-1)对尿素水解及以后的氮转化过程抑制作用较小,有时甚至促进了尿素水解;高浓度腐殖酸(15gkg-1和20gkg-1)则能明显的抑制尿素水解,延长尿素态氮在土壤中的停留时间,增加铵态氮含量,减少硝态氮的生成及氮素损失量,大大提高尿素利用效率。由此可见,腐殖酸不仅是一种脲酶抑制剂,还是一种硝化抑制剂。     

设施菜田不同施氮处理对硝酸盐迁移和积累的影响

在设施菜地条件下,研究了氮肥减施及配施抑制剂处理在黄瓜生长期对土壤NO3--N迁移累积的影响。结果表明,氮肥减施处理可显著降低土壤表层和整个土体的NO3--N含量。常规施氮量时0~40 cm土层的NO3--N含量均高于其它处理,减氮30%后0~40 cm土层未出现NO3--N显著积累现象;氮肥配施抑制剂处理不同程度降低了土壤NO3--N含量,且抑制硝态氮向下层土壤淋失,其中抑制剂组合的效果最好。氮肥配施抑制剂,可以有效控制NO3--N在土壤和植物体内的过量累积,减少硝态氮淋溶损失。     

Involvement of Bradyrhizobium japonicum denitrification in symbiotic nitrogen fixation by soybean plants subjected to flooding

Denitrification by Bradyrhizobium japonicum bacteroids contributes to nitric oxide (NO) production within soybean nodules in response to flooding conditions. However, the physiological relevance of NO production by denitrification in B. japonicum-Glycine max symbiosis is still unclear. In this work, soybean plants were inoculated with B. japonicum strains lacking the nirK or norC genes which encode the copper-containing nitrite reductase and the c-type nitric oxide reductase enzymes, respectively. 14 days flooding increased nodule number of plants inoculated with the WT and norC strains, but not of plants inoculated with the nirK mutant. However, nodule dry weight was not affected by 14 days flooding regardless of the strain used for inoculation. Supporting this observation, individual nodule growth was significantly higher in plants inoculated with nirK than those inoculated with WT or norC after 14 days flooding. Nodule functioning was strongly inhibited by flooding since leghemoglobin content of the nodules induced by any of the strains was significantly decreased after 7 or 14 days flooding compared to control plants. However, this effect was more relevant in nodules of plants inoculated with the WT or norC mutant than in those inoculated with the nirK mutant. Nitrogen fixation was also estimated by analyzing nitrogen content derived from biological nitrogen fixation in shoots, using the ¹⁵N isotope dilution technique. By using this approach, we observed that the negative effect of 14 days flooding on nitrogen fixation was more pronounced in plants inoculated with the norC mutant. However, nitrogen fixation of plants inoculated with nirK showed the highest tolerance to 14 days flooding. These findings allowed us to demonstrate the previously proposed hypothesis which suggests that NO formed by copper-containing nitrite reductase in soybean nodules, in response to flooding, has a negative effect on nitrogenase activity. We propose that inoculation of soybeans with a B. japonicum nirK mutant, which does not produce NO from nitrate, increases the tolerance of symbiotic nitrogen fixation to flooding.     

Changes in forest soil organic matter pools after a decade of elevated CO2 and O3

The impact of rising atmospheric carbon dioxide (CO₂) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O₃) or vary based on species-specific growth responses to elevated CO₂. To understand how projected increases in atmospheric CO₂ and O₃ alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO₂-O₃ Enrichment (FACE) experiment. Tracer amounts of ¹⁵NH₄⁺ were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papyrifera and P. tremuloides-Acer saccharum communities exposed to factorial CO₂ and O₃ treatments. The ¹⁵N tracer and strongly depleted ¹³C-CO₂ were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO₂ relative to ambient CO₂. As main effects, neither CO₂ nor O₃ significantly altered ¹⁵N recovery in SOM. Elevated CO₂ significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO₂ has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O₃ had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO₂ can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response.     

Differential and successive effects of residue quality and soil mineral N on water-stable aggregation during crop residue decomposition

Residue quality has been shown to influence soil water-stable aggregation (WSA) during crop residue decomposition, but there is still little information about its interactive effect with soil mineral N availability. The aim of this study was to determine the effect of soil mineral N on WSA during the decomposition of two high-C/N crop residues (wheat straw with C/N = 125.6 and miscanthus straw with C/N = 311.3). The two crop residues were combined with three mineral N addition rates (0, 60, and 120 mg N kg⁻¹ dry soil). Respiration, soil mineral N content, and WSA (expressed as mean-weight diameter, MWD) were measured on several dates during a 56-d incubation. The effect of decomposing crop residues on WSA followed two phases. (i) Between 0 and 7 d, the increase in WSA was related to intrinsic residue quality with higher decomposability of the wheat straw resulting in higher WSA. (ii) Thereafter, and until the end of the experiment, mineral N addition rates had a predominant but negative influence on WSA. In this second phase, the average MWD of residue-treated soils was 0.92, 0.55, and 0.44 mm for the 0, 60 and 120 mg N kg⁻¹ dry soil addition rates, respectively. Mineral N addition which did result in higher crop residue decomposition did not lead to higher WSA. WSA during crop residue decomposition is therefore not simply positively related to the induced microbial activity, and changes in microbial community composition with differential effects on WSA must be involved. The impact of high-C/N crop residues inputs on WSA, initially assumed to be low, could actually be strong and long-lasting in situations with low soil mineral N content.     

Effects of fragmentation on pollinator assemblage, pollen limitation and seed production of Mediterranean myrtle (Myrtus communis)

Pollinator assemblages may shift as a consequence of the destruction and fragmentation of natural habitats. The scarcity of mates and pollinators can lead plant populations to suffer from pollen limitation and a decrease in reproductive performance within fragmented areas. We studied the shift in pollinator assemblages along with pollen limitation and seed production patterns in the Mediterranean shrub Myrtus communis. Our study included six populations contrasting in patch and population size (Large vs. Small) within a fragmented landscape characterized by ∼1% of potential forest coverage. The breeding system in Myrtus communis was self-compatible, but compared with natural pollination, fruit set increased with pollen addition (quantity limited), and seed set (brood size) increased with outcross pollen addition (quality limited). While the pollinator assemblage in Large patches was taxonomically diverse, it was almost monopolized by honeybees in Small patches, where visitation rates were highest and wild bee species were almost absent. In general, Small populations were less pollen limited for fruit set than Large populations, particularly those that received the highest rates of honeybee visits. However, despite differences in fragmentation and pollinators between Large and Small populations, seed production patterns (brood size and seed mass) were rather similar among them, in agreement with similar pollen limitation levels found for brood size. A higher susceptibility of native pollinators to the presence of honeybee hives was found in Small patches, suggesting that the pollinator assemblage may be severely altered when fragmentation occurs in combination with beekeeping. We discuss its implications and effects on plant reproduction in fragmented areas.     

Compartmentalization of the soil animal food web as indicated by dual analysis of stable isotope ratios (N/N and C/C)

The soil animal food web has become a focus of recent ecological research but trophic relationships still remain enigmatic for many taxa. Analysis of stable isotope ratios of N and C provides a powerful tool for disentangling food web structure. In this study, animals, roots, soil and litter material from a temperate deciduous forest were analysed. The combined measurement of δ¹⁵N and δ¹³C provided insights into the compartmentalization of the soil animal food web. Leaf litter feeders were separated from animals relying mainly on recent belowground carbon resources and from animals feeding on older carbon. The trophic pathway of leaf litter-feeding species appears to be a dead end, presumably because leaf litter feeders (mainly diplopods and oribatid mites) are unavailable to predators due to large size and/or strong sclerotization. Endogeic earthworms that rely on older carbon also appear to exist in predator-free space. The data suggest that the largest trophic compartment constitutes of ectomycorrhizal feeders and their predators. Additionally, there is a smaller trophic compartment consisting of predators likely feeding on enchytraeids and potentially nematodes.     

FACE对三系杂交籼稻汕优63根系活性影响的研究

2005、2006年利用我国惟一的农田开放式空气CO2浓度增高（FACE）研究平台,设计施N量为125kg·hm^-2（LN）、250kg·hm^-2（NN）处理,研究大气CO2浓度比对照高200umol·mol^-1的FACE处理对三系杂交籼稻汕优63根系活性的影响。结果表明：（1）FACE处理使汕优63不同生育时期单位干质量根系的总吸收面积、活跃吸收面积、α-萘胺氧化量等根系活性指标均极显著小于对照。由于FACE处理促进汕优63根系发生量的大幅度增加,因此分蘖期、拔节期其单穴根系活性与对照多无明显差异,到抽穗期FACE处理单穴根系活性显著大于对照;（2）拔节期、抽穗期汕优63每穴的不定根数、不定根总长度、根系体积、根干质量与单位干质量根系活性的关系密切,根量越大单位于质量根系活性越低;（3）不同生育时期汕优63植株含氮率与单位干质量的根系活性多呈正相关,植株碳氮比与单位干质量的根系活性多呈负相关;（4）FACE处理汕优63根系生长量大、植株含氮率低、碳氮比高等可能是造成其单位干质量根系活性低于对照的重要原因。