Aggregate-related changes in network patterns of nematodes and ammonia oxidizers in an acidic soil

Nitrification plays a central role in global nitrogen cycle, which is affected by biological interaction between soil microfauna and microorganisms. However, the complexity of soil biotic communities made it difficult to reveal organizational principles of the community and the interactions among species. Here, we used the network analysis to decipher the interactions between nematodes and ammonia oxidizers within aggregate fractions under 10-year manure application, and examine their associations with soil variables and potential nitrification activity (PNA). Three aggregate fractions included large macroaggregates (>2000 μm, LA), small macroaggregates (250–2000 μm, SA), and inter-aggregate soil and space (<250 μm, IA). Aggregate factions showed a remarkable effect on association networks of nematodes and ammonia oxidizers. The average connectivity (avgK) and the number of edges in overall networks increased with increasing aggregate sizes, while the average geodesic distance (GD) followed the opposite trend. The LA network could be viewed as a better organized or a better operational soil food web with more functional interrelated members than the SA and IA networks. The modules related to PNA were significantly correlated and clustered together as meta-modules in networks of aggregate fractions. The role-shifts prevailed among the network members such as significant module memberships (MMs) and generalist/specialist operational taxonomic units (OTUs). A half of shared nodes were further identified as shared MMs, dominated by ammonia-oxidizing bacteria (AOB) especially for Nitrosospira cluster 3a and 10. Soil pH could explain partly the shift of module hubs in different networks, while grazing by bacterivores might account for three exclusively connecters related to Nitrososphaera clusters 1.1. The strongly coupled modules correlated positively to pH and total carbon (TC), regardless of aggregate fractions. The network analysis approach provided new insights into potential importance of network interactions between nematodes and ammonia oxidizers in soil nitrogen cycling.     

Desiccation and cracking behaviour of clay layer from slurry state under wetting-drying cycles

Laboratory tests were conducted to investigate the effect of wetting-drying (W-D) cycles on the initiation and evolution of cracks in clay layer. Four identical slurry specimens were prepared and subjected to five subsequent W-D cycles. The water evaporation, surface cracks evolution and structure evolution during the W-D cycles were monitored. The effect of W-D cycles on the geometric characteristics of crack patterns was analyzed by image processing. The results show that the desiccation and cracking behaviour was significantly affected by the applied W-D cycles: the measured cracking water content θ_c, surface crack ratio R_sc and final thickness h_f of the specimen increased significantly in the first three W-D cycles and then tended to reach equilibrium; the formed crack patterns after the second W-D cycle were more irregular than that after the first W-D cycle; the increase of surface cracks was accompanied by the decrease of pore volume shrinkage during drying. In addition, it was found that the applied W-D cycles resulted in significant rearrangement of specimen structure: the initially homogeneous and non-aggregated structure was converted to a clear aggregated-structure with obvious inter-aggregate pores after the second W-D cycle; the specimen volume generally increased with increasing cycles due to the aggregation and increased porosity. The image analysis results show that the geometric characteristics of crack pattern were significantly influenced by the W-D cycles, but this influence was reduced after the third cycle. This is consistent with the observations over the experiment, and indicates that the image processing can be used for quantitatively analyzing the W-D cycle dependence of clay desiccation cracking behaviour.     

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.     

不同有机厩肥输入量对土壤团聚体有机碳组分的影响

土壤是重要的有机碳库,其微小变化可能引起大气CO2浓度水平的较大变异。土壤团聚体对土壤有机碳具有物理保护作用。有机厩肥的输入既可以提高土壤有机碳含量,又可以促进土壤团聚体的形成,对土壤有机碳的截获和保持有重要意义。本实验采用湿筛的方法分离土壤团聚体,并对团聚体进行有机碳组分分离。通过对连续8年施加不同量有机厩肥试验的研究发现:适量的有机厩肥施用可以显著地提高土壤的平均质量直径(MWD),改善土壤结构;过量施用有机厩肥则明显降低了>2 000μm团聚体含量。潮棕壤有机碳主要分布在250～53μm和2 000～250μm团聚体中,两者相加约占有机碳全量的73.7%～78.5%。并且随着有机碳输入量的增加,土壤有机碳主要贮存在2 000～250μm团聚体中。有机厩肥的施加明显地加快了>2 000μm团聚体的更新速率。土壤轻组分有机碳含量也随有机厩肥输入量的增加而不断增加,高量有机厩肥下占全量的22.1%。土壤固定有机碳的能力有限,存在明显的等级饱和现象。因此,在有机质匮乏的土壤施用有机肥意义重大,应尽量减少向高有机质土壤输入有机碳。     

Transitioning from standard to minimum tillage: Trade-offs between soil organic matter stabilization, nitrous oxide emissions, and N availability in irrigated cropping systems

Few studies address nutrient cycling during the transition period (e.g., 1–4 years following conversion) from standard to some form of conservation tillage. This study compares the influence of minimum versus standard tillage on changes in soil nitrogen (N) stabilization, nitrous oxide (N₂O) emissions, short-term N cycling, and crop N use efficiency 1 year after tillage conversion in conventional (i.e., synthetic fertilizer-N only), low-input (i.e., alternating annual synthetic fertilizer- and cover crop-N), and organic (i.e., manure- and cover crop-N) irrigated, maize–tomato systems in California. To understand the mechanisms governing N cycling in these systems, we traced ¹⁵N-labeled fertilizer/cover crop into the maize grain, whole soil, and three soil fractions: macroaggregates (>250 μm), microaggregates (53–250 μm) and silt-and-clay (<53 μm). We found a cropping system effect on soil N_new (i.e., N derived from ¹⁵N-fertilizer or -¹⁵N-cover crop), with 173 kg N_new ha⁻¹ in the conventional system compared to 71.6 and 69.2 kg N_new ha⁻¹ in the low-input and organic systems, respectively. In the conventional system, more N_new was found in the microaggregate and silt-and-clay fractions, whereas, the N_new of the organic and low-input systems resided mainly in the macroaggregates. Even though no effect of tillage was found on soil aggregation, the minimum tillage systems showed greater soil fraction-N_new than the standard tillage systems, suggesting greater potential for N stabilization under minimum tillage. Grain-N_new was also higher in the minimum versus standard tillage systems. Nevertheless, minimum tillage led to the greatest N₂O emissions (39.5 g N₂O–N ha⁻¹ day⁻¹) from the conventional cropping system, where N turnover was already the fastest among the cropping systems. In contrast, minimum tillage combined with the low-input system (which received the least N ha⁻¹) produced intermediate N₂O emissions, soil N stabilization, and crop N use efficiency. Although total soil N did not change after 1 year of conversion from standard to minimum tillage, our use of stable isotopes permitted the early detection of interactive effects between tillage regimes and cropping systems that determine the trade-offs among N stabilization, N₂O emissions, and N availability.     

不同有机物料对连作大豆土壤养分及团聚体组成的影响

为探究不同有机物料对连作大豆土壤养分和团聚体组成的影响,采用大田试验方法,研究了在连作7年条件下,腐熟鸡粪(JF)、玉米秸秆(JG)和木耳菌渣(JZ)三种不同类型有机物料对连作大豆土壤养分、产量、团聚体组成和稳定性的影响。结果表明:与CK相比有机物料能有效提高连作大豆土壤养分含量,显著影响土壤团聚体组成,0.25~2 mm粒级团聚体含量明显增加。畜禽粪便类的鸡粪(JF)对碱解氮的提高作用最明显,较CK提高13.85%;天然类的秸秆(JG)对速效磷含量提高作用显著,较CK提高4.13%;JF对提高速效钾含量效果最好,较CK提高8.97%;JG对提高有机质含量效果最好,较CK提高7.28%。半腐解类的菌渣(JZ)较CK相比土壤养分含量也有明显的提高,但不如JF和JG效果明显。有机物料能提高连作大豆产量,JF效果显著。所有有机物料对改善土壤团聚体组成,增加0.25 mm水稳性团聚体含量效果均比较显著,对土壤水稳性团聚体平均重量直径以及土壤团聚体的稳定性指数有显著提高作用。     

不同施肥条件下微生物对棕壤团聚体和碳分布的影响

以北京市延庆县绿富隆有机肥蔬菜研究基地长期定位肥料试验地为试验平台,利用湿筛法获得不同粒级的团聚体,通过16SrDNA-PCR-DGGE技术进行测序分析,研究不同施肥条件下微生物群落对棕壤土团聚体和碳分布的影响。结果表明,有机肥(OF)处理的0.25～2 mm水稳性团聚体增加,增加幅度为109.0%;0.053～0.25 mm和小于0.053 mm粒级团聚体的含量均下降,与CK(不施肥)相比分别下降了31.9%和142.1%。OF处理对土壤各粒级团聚体中碳含量均有显著提高,与CK相比,提高15.2%～46.9%,其中大于2 mm团聚体中碳含量提高了46.9%。棕壤碳含量与大于2 mm粒级团聚体含量呈正相关;与0.25～2 mm粒级团聚体呈极显著正相关;与0.053～0.25 mm粒级团聚体含量呈极显著负相关;与小于0.053 mm粒级团聚体呈显著负相关。     

Carbohydrate composition in relation to structural stability, compactibility and plasticity of two soils in a long-term experiment

The effects of tillage on soil organic carbon content, carbohydrate content, monosaccharide composition, aggregate stability, compactibility and plasticity were investigated in a field experiment on a gleysol and on a cambisol under winter barley in South-East Scotland. Two long-term treatments (direct drilling and conventional mouldboard ploughing for 22 years) were compared with short-term direct drilling and broadcast sowing plus rotavation for 5 years. Carbohydrate released sequentially to cold water, hot water, 1.0 M HCl and 0.5 M NaOH was determined after hydrolysis as reducing sugar equivalent to glucose in both fresh and air-dried samples. All other measurements were made on dry soils only. About 3% of the soluble carbohydrate was extracted by cold water, 10% by hot water, 12% by HCl and 75% by NaOH from both the dry and fresh soils. The total reducing sugars of the fractions were proportional to the total organic carbon determined by dichromate oxidation or C analysis. Organic carbon and carbohydrates were concentrated near the surface of the direct drilled soil, but were more uniformly distributed with depth in the ploughed soil. The surface soil under direct drilling was more stable, less compactible and had greater plasticity limits than under ploughing. However, particle size distributions were unaffected by tillage so that differences in soil properties were attributed to differences in the quantity and quality of organic matter. Differences in compactibility, structural stability and plasticity limits between depths and tillage treatments correlated with total carbon and with total carbohydrates. The hot water extractable carbohydrate fraction correlated best with aggregate stability and the NaOH fraction correlated best with compactibility and plastic limit. Both fractions were greatest in the long-term direct drilled soil. The hot water fraction had a galactose plus mannose over arabinose plus xylose ratio of 1.0–1.6 in comparison to 0.4–0.7 in the NaOH fraction indicating that the microbial contribution within the hot water-soluble fraction was the greater. The hot-water fraction was likely to contain more exocellular microbial polysaccharides involved in the stabilizing of soil aggregates. The hot-water and NaOH carbohydrate fractions may be good indicators of soil organic matter quality relevant to the preservation of good soil physical conditions.     

长期施肥对红壤性水稻土团聚体特征的影响

利用干筛法研究了红壤性水稻土连续施肥23年后,各粒径土壤团聚体含量及其特征值变化.结果表明:施用有机肥(NPKM、OM)与CK相比,可提高粒径3～5 mm土壤团聚体含量25.4%～24.6%,并分别降低粒径0.05～0.25 mm、＜0.05mm团聚体含量70.6%～71.1%和113.6%～121.7%,差异达显著水平;单施化肥处理(N、NP、NPK)与CK相比,各粒径土壤团聚体含量无明显差异,表明施肥对红壤性水稻土团聚体的影响主要受有机肥的控制.施用有机肥显著提高红壤性水稻土团聚体的平均重量直径(MWD)、几何平均直径(GMD),而降低团聚体的分形维D值:水稻土团聚体的MWD、GMD与＞3 mm团聚体含量和水稻年产量存在显著的正相关,而团聚体分形维D值则与＞3 mm团聚体含量和水稻年产量存在显著的负相关,因而土壤团聚体的MWD、GMD和分形维D值可较客观地反映土壤肥力的状态.不同施肥处理的水稻年产量与粒径3～5 mm团聚体含量呈显著正相关而与0.05～0.25 mm、＜0.05 mm团聚体含量呈显著负相关,表明红壤性水稻土团聚体干筛分析条件下,粒径3～5 mm、0.05～0.25 mm、＜0.05 mm团聚体含量可作为红壤性水稻土土壤肥力表征的物理性指标.     

Inherent bias in using aggregate CPUE to characterize abundance of fish species assemblages

We have analyzed the practice of assessing an assemblage of fish species in a multispecies fishery on the basis of aggregate catch per unit effort (CPUE), which is the summed catch of all species per unit of effort. We show that at the onset of fishing or of a large positive or negative change in fishing effort, aggregate CPUE will be hyper-responsive, that is, relative change of aggregate CPUE will be greater than that of aggregate abundance. We also show that as the fishery reaches equilibrium, the aggregate CPUE in most circumstances will continue to be hyper-responsive, with a greater relative change from its value at the start than the aggregate abundance. However, there are less likely circumstances in which the aggregate CPUE will be hyper-stable compared to aggregate abundance. The circumstances leading to hyper-responsiveness or hyper-stability depend on the distribution of productivity and fishery vulnerability parameters among the species in the aggregation.