Controls of temporal and spatial variability of methane uptake in soils of a temperate deciduous forest with different abundance of European beech (Fagus sylvatica L.)

Aerated forest soils are a significant sink for atmospheric methane (CH₄). Soil properties, local climate and tree species can affect the soil CH₄ sink. A two-year field study was conducted in a deciduous mixed forest in the Hainich National Park in Germany to quantify the sink strength of this forest for atmospheric CH₄ and to determine the key factors that control the seasonal, annual and spatial variability of CH₄ uptake by soils in this forest. Net exchange of CH₄ was measured using closed chambers on 18 plots in three stands exhibiting different beech (Fagus sylvatica L.) abundance and which differed in soil acidity, soil texture, and organic layer thickness. The annual CH₄ uptake ranged from 2.0 to 3.4 kg CH₄-C ha⁻¹. The variation of CH₄ uptake over time could be explained to a large extent (R² = 0.71, P < 0.001) by changes in soil moisture in the upper 5 cm of the mineral soil. Differences of the annual CH₄ uptake between sites were primarily caused by the spatial variability of the soil clay content at a depth of 0-5 cm (R² = 0.5, P < 0.01). The CH₄ uptake during the main growing period (May-September) increased considerably with decreasing precipitation rate. Low CH₄ uptake activity during winter was further reduced by periods with soil frost and snow cover. There was no evidence of a significant effect of soil acidity, soil nutrient availability, thickness of the humus layer or abundance of beech on net-CH₄ uptake in soils in this deciduous forest. The results show that detailed information on the spatial distribution of the clay content in the upper mineral soil is necessary for a reliable larger scale estimate of the CH₄ sink strength in this mixed deciduous forest. The results suggest that climate change will result in increasing CH₄ uptake rates in this region because of the trend to drier summers and warmer winters.     

Tree identity surpasses tree diversity in affecting the community structure of oribatid mites (Oribatida) of deciduous temperate forests

The role of tree diversity and identity as determinants of soil animal community structure is little understood. In a mature deciduous forest dominated by beech we identified clusters of one, two and three tree species of beech, ash and lime allowing to investigate the role of tree species diversity and identity on the density and community structure of oribatid mites. To relate oribatid mite community structure to environmental factors we measured leaf litter input, fine root biomass, mass of organic layers, topsoil pH and C and N content. We expected oribatid mite density to increase with increasing tree diversity, but we expected the effects of tree species identity to override effects of tree diversity. Further, we hypothesized the density of oribatid mites to be reduced by the presence of beech but increased by the presence of lime and ash. As expected tree diversity little affected oribatid mite communities, whereas tree species identity strongly altered density and community structure of oribatid mites. However, in contrast to our expectations the density of oribatid mites was highest in presence of beech indicating that many oribatid mite species benefit from the presence of recalcitrant litter forming thick organic layers. Especially Oppioidea benefited from the presence of beech presumably due to an increased availability of food resources such as fungi and nematodes. Lower density of oribatid mites in monospecific clusters of lime and ash suggests that oribatid mites did not benefit from high quality litter of these species. Notably, large and strongly sclerotized oribatid mite species, such as Steganacarus magnus and Chamobates voigtsi, benefited from the presence of ash and lime. Presumably, these large species better resist harsh microclimatic conditions in shallow organic layers.     

温带牧草地牧草干物质量和N素产量的空间格局的时间稳定性

The objectives of the present study were to examine the spatial patterns of sward dry matter （DM） and nitrogen （N） yields in a grass silage field at first, second, and third cuts over a 3-year period; quantify their temporal stabilities with temporal stability maps; and assess the potential for site-specific management in each pasture-growing period using classified management maps. At cut 1, the spatial patterns of DM and N yields proved to be well defined and temporally stable and were likely to be due to differences in the net N mineralization rates across the field during spring. In contrast, at cut 2, the patterns of DM production were patchy and temporally unstable. It was concluded that, in principle, a simple site-specific approach to N fertilization would be possible in this field during spring at cut 1. At later harvests, the rationale for a site-specific approach to fertilizer management was less clear with logistics appearing to be more complex and less feasible.     

Effects of the annual invasive plant Impatiens glandulifera on the Collembola and Acari communities in a deciduous forest

Invasive plants can disturb interactions between soil organisms and native plants and thereby alter ecosystem functions and/or reduce local biodiversity. Collembola and Acari are the most abundant microarthropods in the leaf litter and soil playing a key role in the decomposition of organic material and nutrient cycling. We designed a field experiment to examine the potential effects of the annual invasive plant Impatiens glandulifera on species diversity, abundance and community composition of Collembola and Acari in leaf litter and soil in a deciduous forest in Switzerland. Leaf litter and soil samples were obtained from plots invaded by I. glandulifera since 6 years, from plots in which the invasive plant had been removed for 4 years and from plots which were not yet colonized by the invasive plant. The 45 leaf litter and soil samples were equally distributed over three forest areas, which were differently affected by a wind throw 12 years prior to sampling representing a natural gradient of disturbance. Collembola species richness and abundance in the leaf litter and soil samples were not affected by the presence of the invasive plant. However, the species composition of Collembola was altered in plots with I. glandulifera. The abundance of leaf-litter dwelling Acari was increased in invaded plots compared to the two other plot types. Furthermore, the presence of the invasive plant shifted the composition of Acari individuals belonging to different groups. Our field experiment demonstrates that an annual invasive plant can affect microarthropods which are important for nutrient cycling in various ecosystems.     

畦面微地形时空变异分布状况试验研究

畦面微地形时空变异分布状况对畦灌性能的影响较为显著,该文依据从典型试验畦块获得的畦面相对高程实测数据,借助统计学方法分析评价田间耕作措施和地面灌溉对畦面微地形时空变异分布状况的影响。结果表明：农田机耕、围畦打埂等田间耕作措施对畦面微地形整体分布状态和畦面相对高程空间分布状况的影响显著;而地面灌溉虽对畦面相对高程空间分布状况有一定影响,但对畦面微地形整体分布状态的影响却较小;田间耕作和地面灌溉等人为生产活动对较小规格畦块的畦面微地形时空变异分布状况的影响明显一些,围畦打埂和入畦单宽流量是影响土地精平后畦面微地形分布状况发生变化的主要影响因素;对较大规格的畦块,除上述影响因素外,还应关注机耕作业留下的车轮压痕对畦面平整状况产生的负面作用。     

Leaf-cutting ant (Atta Sexdens) and nutrient cycling: deep soil inorganic nitrogen stocks, mineralization, and nitrification in Eastern Amazonia

Nest excavation and agricultural activities of the leaf-cutting ant Atta sexdens create complex belowground heterogeneity in secondary forests of Eastern Amazonia. We examined the effects of this heterogeneity on inorganic-N stocks, net mineralization, and net nitrification to test the hypothesis that the bulk soil of the nests has higher net rates of mineralization and nitrification than soil that was not affected by the influences of ant nests, throughout the profile. This study was conducted in a secondary forest at Fazenda Vitoria, near Paragominas in the Eastern Brazilian Amazon, where a previous study showed that the bulk soil of ant nests had elevated NO₃⁻. The results of the inorganic-N measurements were consistent with the previous study, showing elevated NO₃⁻ deep in the soil profile of the nests. However, neither net mineralization nor net nitrification were significantly greater at depth in the mineral soil of the nests compared to soil that was not influenced by nests (P=0.05), although variability was higher in the nest soil. These results suggest that the NO₃⁻ may have diffused into the surrounding mineral from the N-rich organic matter buried by the ants in chambers within the deep soil.     

Vertical distribution of Collembola (Hexapoda) and their food resources in organic horizons of beech forests

 Micro-samples of the surface organic horizons of 13 beech forests in Belgium were fixed immediately after collection in ethanol. Collembola (6255 animals) were sorted directly from micro-samples in the laboratory using a dissecting microscope, while the litter/soil matrix was analysed semi-quantitatively. The vertical distribution of Collembolan species was studied by correspondence analysis. Gut contents of animals were examined under a light microscope and their composition was compared with that of the matrix. A consistent association was found between the vertical distribution of gut contents and that of food resources in the immediate proximity of animals. Species differed in their feeding habits but most of them ingested a wide spectrum of food items. Plasticity in the food regime according to depth could be demonstrated in members of the Onychiuridae family. Received: 11 January 1999     

Analysis of yield variability in winter wheat due to temporal variability,and nitrogen and phosphorus fertilization

Abstract

Field average based recommendations have been a common practice for recommending the major crop nutrients nitrogen (N) and phosphorus (P). The problem is yield will not be the same from year to year with application of the same amount of recommended rate of fertilizer. The objectives of this study were to demonstrate how recommendations generated using nutrient response experiments were dynamic; and to assess the relative contribution of temporal variability, N and P fertilizers on winter wheat grain yield and N concentration. Twelve factorial combinations of four N (0, 56, 112, and 168 kg ha^?1) and three P (0, 14.5, and 29 kg P ha^?1) rates were evaluated in a randomized complete block design with three replications at Perkins, Oklahoma. To address the first objective, ANOVA and orthogonal polynomial contrasts were used. To address the second objective, a ten predictor variable multiple linear regression model with two quantitative variables and their interaction (N, P and N×P) and seven-year variables was evaluated and a reduced model containing seven variables was generated. Wheat grain yield showed three distinct responses to N rates: Linear, quadratic and no response. These individual year data show that it is not always appropriate to use results of nutrient response experiments to estimate next year's N fertilizer requirement due to apparent temporal variability in the results. Wheat only responded to P during the first two years of the study. The reduced model from the regression analysis revealed that most of the variability in grain yield was accounted for by five individual indicator years and N only. High variability across years in grain yield and fertilizer (N and P) response, even between years of similar grain yield, is an indication of a given season's production dependence on factors other than N and P.     

基于土壤电导率时空变异性的管理分区技术研究

A coastal saline field of 10.5 ha was selected as the study site and 122 bulk electrical conductivity （ECb） measurements were performed thrice in situ in the topsoil （0-20 cm） across the field using a hand held device to assess the spatial variability and temporal stability of the distribution of soil electrical conductivity （EC）, to identify the management zones using cluster analysis based on the spatiotemporal variability of soil EC, and to evaluate the probable potential for sitespecific management in coastal regions with conventional statistics and geostatistical techniques. The results indicated high coefficients of variation for topsoil salinity over all the three samplings. The spatial structure of the salinity variability remained relatively stable with time. Kriged contour maps, drawn on the basis of spatial variance structure of the data, showed the spatial trend of the salinity distribution and revealed areas of consistently high or consistently low salinity, while a temporal stability map indicated stable and unstable regions. On the basis of the spatiotemporal characteristics, cluster analysis divided the site into three potential management zones, each with different characteristics that could have an impact on the way the field was managed. On the basis of the clearly defined management zones it was concluded that coastal saline land could be managed in a site-specific way.     

Soil CO2 efflux in a temperate deciduous forest: Environmental drivers and component contributions

Soil CO₂ efflux is a large component of total respiration in many ecosystems. It is important to understand the environmental controls on soil CO₂ efflux, in order to evaluate potential responses of ecosystems to climate change. This study investigated the relationship between total soil CO₂ efflux and soil temperature, soil moisture and solar radiation on an interannual basis for a plot of temperate deciduous ancient semi-natural woodland at Wytham Woods in central southern England. We also aimed to quantify the contribution of soil organic matter decomposition (SOM), root-and-rhizosphere respiration, and mycorrhizal respiration components to total soil CO₂ efflux, and determine their environmental correlates. Total soil CO₂ efflux was measured regularly from April 2006 to December 2008 and found to average 4.1 Mg C ha⁻¹ yr⁻¹ in both 2007 and 2008. In addition, we applied a recently developed approach to partition the efflux into SOM, root-and-rhizosphere, and mycorrhizal components in situ using mesh bags. SOM decomposition, root-and-rhizosphere, and mycorrhizal respiration were estimated to contribute 70 ± 6%, 22 ± 6% and 8 ± 3% of total soil CO₂ efflux respectively, equating to 3.0 ± 0.3, 0.9 ± 0.2 and 0.3 ± 0.1 Mg C ha⁻¹ yr⁻¹. In order to avoid the effect of temporal correlation between variables caused by seasonality, we investigated interannual variability by examining the relationship between CO₂ flux anomalies and anomalies in environmental variables. Variation in soil temperature explained 50% of the interannual variance in soil CO₂ efflux, and soil moisture a further 18% of the residual variance. Solar radiation, as a proxy for plant photosynthesis, had no significant effect on total soil CO₂ efflux, but was positively correlated with root-and-rhizosphere respiration, and mycorrhizal respiration. The relationship between anomalies in soil CO₂ efflux and soil temperature was highly significant, with a sensitivity of 0.164 ± 0.023 μmol CO₂ m⁻² s⁻¹ °C⁻¹. For mean peak summer efflux rates (2.03 μmol CO₂ m² s⁻¹), this is equivalent to 8% per °C, or a Q₁₀ temperature sensitivity of 2.2 ± 0.2. We demonstrate the utility of an anomaly analysis approach and conclude that soil temperature is the key driver of total soil CO₂ efflux primarily through its positive relationship with SOM-decomposition rate.     

Spatial and temporal patterns of water-extractable organic carbon (WEOC) of surface mineral soil in a cool temperate forest ecosystem

Water-extractable organic carbon (WEOC) drives the C and N cycles in forest ecosystems via microbial activity. However, few studies have considered both then spatial and temporal patterns of WEOC in forest soils. We investigated the spatial and temporal variation in WEOC along a topographic sequence in a cool temperate deciduous forest. The concentrations of WEOC, carbohydrates, total phenols, and other organics were 126±51, 40±15, 1.5±0.5 and 85±43 mg C kg dry soil⁻¹, respectively. Carbohydrates and phenols accounted for 33±11 and 1.5±1.0% of WEOC, respectively. The effect of season on the WEOC concentration was stronger than that of slope position the growing season, although most of the soil properties varied markedly with slope position. The concentration of carbohydrates in WEOC showed similar seasonal patterns across slope positions. The carbohydrate concentration peaked in May and August. The results suggest that carbohydrates are controlled by the recent production of C, rather than by organic C that has accumulated in soil.     

Direct and indirect effects of tree diversity drive soil microbial diversity in temperate deciduous forest

We investigated the link between aboveground and belowground diversity in temperate deciduous forest ecosystems. To this end, we determined the effects of the tree species composition on the biomass and composition of the soil microbial community using phospholipid fatty acid (PLFA) profiles in the Hainich National Park, a deciduous mixed forest on loess over limestone in Central-Germany. We investigated the effects of the leaf litter composition on the microbial community, hypothesizing that distinctive leaf litter compositions increase signature PLFAs. In addition, we studied the impact of clay content, pH and nutrient status of the soil on the microbial community in different surface soil layers. Consequently, soil was sampled from depths of 0-5 cm, 5-10 cm and 10-20 cm. Plots with highest leaf litter diversity had the largest total amounts of fatty acids, but only PLFA 16:1ω5, which is a common marker for arbuscular mycorrhizal fungi, was significantly increased. In the uppermost soil layer, the pH explained most of the variance in microbial composition. In the deeper surface soil layers, nutrients such as carbon, nitrogen and phosphorus determined the microbial abundances and composition. Our results suggest that the soil microbial community is mainly indirectly influenced by aboveground diversity. Changes in soil pH or the soil nutrient status that are driven by specific plant traits like leave litter quality drive these indirect changes. Specific direct interactions are most reasonable for mycorrhizal fungi.     

武汉市汉南区土壤有机质和养分时空变异研究

运用经典统计学及地统计学方法,研究汉南区土壤有机质和养分含量的空间变异特征。结果表明:碱解氮、有效磷、速效钾和有机质含量为中等水平,均服从正态分布;碱解氮和有机质符合球形模型,有效磷和速效钾符合指数模型。碱解氮和有机质的块金效应均在25%~75%之间,表明系统具有中等空间相关性。有效磷和速效钾块金效应均25%,表明系统具有强烈空间相关性。在地理信息系统(GIS)支持下,通过Kriging插值研究发现4种养分空间分布大体趋势是,东荆街办养分含量高,湘口街办和纱帽街办次之,邓南街办最低。从1980到2010年,pH值基本没有变化,有机质有降低的趋势,有效磷存在大幅提升,速效钾含量先大幅降低,后有所回升。培肥措施应为补施有机肥,稳施氮肥,优化磷、钾肥。     

Soil inorganic nitrogen composition and plant functional type determine forage crops nitrogen uptake preference in the temperate cultivated grassland,Inner Mongolia

ABSTRACT

Plant nitrogen (N)-acquisition strategy affects soil N availability, community structure, and vegetation productivity. Cultivated grasslands are widely established to improve degraded pastures, but little information is available to evaluate the link between N uptake preference and forage crop biomass. Here an in-situ ¹⁵N labeling experiment was conducted in the four cultivated grasslands of Inner Mongolia, including two dicots (Medicago sativa and Brassica campestris) and two monocots (Bromus inermis and Leymus chinensis). Plant N uptake rate, shoot- and root biomass, and concentrations of soil inorganic-N and microbial biomass-N were measured. The results showed that the root/shoot ratios of the dicots were 2.6 to 16.4 fold those of the monocots. The shoot N concentrations of the dicots or legumes were 40.6% to 165% higher than those of the monocots or non-legumes. The four forage crops in the cultivated grassland preferred to uptake more NO₃^?-N than NH₄⁺-N regardless of growth stages, and the NH₄⁺/NO₃^? uptake ratios were significantly lower in the non-legumes than in the legumes (p < 0.05). Significant differences in the NH₄⁺-N rather than NO₃^?-N uptake rate were observed among the four forages, related to plant functional types and growth stages. The NH₄⁺ uptake rate in the perennial forages exponentially decreased with the increases in shoot-, root biomass, and root/shoot ratio. Also, the plant NH₄⁺/NO₃^? uptake ratio was positively correlated with soil NH₄⁺/NO₃^? ratio. Our results suggest that the major forage crops prefer to absorb soil NO₃^?-N, depending on soil inorganic N composition and belowground C allocation. The preferential uptake of NO₃^?-N by forages indicates that nitrate-N fertilizer could have a higher promotion on productivity than ammonium-N fertilizer in the semi-arid cultivated grassland.     

东北地区旱地土壤全氮空间变异性对幅度拓展的响应

土壤氮素不仅是作物必需的营养元素,还是重要的生态元素。不同幅度上土壤全氮含量空间变异的研究是正确评价土壤质量和精确估算土壤氮库的基础。基于中国东北地区第二次土壤普查的1 041个旱地剖面数据,设定三种类型的幅度,即行政区幅度(县、市、省和大区)、土壤分类幅度(土属、亚类、土类和土纲)和土壤类型(地带性土壤和非地带性土壤)-行政区幅度,研究了该区旱地土壤表层(0～20 cm)和剖面(0～100 cm)全氮含量的变异性及其对幅度拓展的响应。结果表明,三种类型幅度上,表层和剖面的平均土壤全氮含量变异性均随着幅度的拓展而增大。在行政区幅度,旱地面积或行政区面积与平均土壤全氮含量变异系数具有较强的对数关系。在土壤分类幅度上,不同土纲的平均土壤全氮含量的变异随幅度变化的趋势、变幅等均存在差异。在土壤类型—行政区幅度,地带性土壤与非地带性土壤的全氮含量随幅度的变化趋势一致,但变化速率有差别,旱地面积与平均非地带性土壤全氮含量变异系数有较强的对数关系。该结果可为东北地区不同幅度下基于土壤类型聚类的旱地土壤氮库估算、土壤资源调查时采样数目的确定提供理论指导。     

Microbial biomass nitrogen pool in soils from a warm temperate grassland,and from deciduous and evergreen forests in Chiba,central Japan

We evaluated the status of the microbial biomass N pool in grassland, and in deciduous and evergreen forest soils in Chiba, central Japan. Microbial biomass N, a labile fraction of total N in the soil, ranged from 6.96 g N m^-2 (15 cm depth) in the grassland to 24.8 g in the deciduous and 20.7 g in the evergreen soils, on a landscape basis. Thus the pattern in the grassland and in the forest soils differed. The N flush measured by a fumigation-incubation method indicated that in the grassland soil microbial biomass N was underestimated by a factor of 2.6 compared with the results from a fumigation-extraction method, because of heavy N immobilization in the microbial biomass. This was in contrast to results from the forest soils, which did not immobilize N. Thus, the forest soils were in a steady-state condition compared with the grassland which formed a seral phase in the ecological succession. Simple correlation coefficients indicated a significant positive relationship between biomass N and organic C in the soil and the N concentration in the litter, the main component of organic matter in the soils of the three ecosystems.     

河北省表层土壤有机碳和全氮空间变异特征性及影响因子分析

【目的】在陆地生态系统中, 土壤全氮和有机碳是重要的生态因子。本研究基于土壤调查获得大量土壤剖面的空间和属性信息,研究河北的土壤有机碳和全氮的空间分布特征,为河北的土壤养分监测和管理提供科学依据,同时也为其他类似地区土壤采样提供参考,减少采样成本。【方法】运用传统统计学和地统计学分析方法,以变异函数为工具,初步分析了河北土壤全氮和有机碳的空间变异特征,并应用普通克立格法和回归克里格法进行插值, 得出全氮和有机碳含量的分布格局。【结果】研究区土壤有机碳和全氮的平均值分别为15.25 g/kg和1.23 g/kg,变异系数分别为0.73和0.63,属于中等强度变异。经对数转换后,土壤有机碳和全氮均符合正态分布。选择球状模型作为土壤有机碳和全氮的半方差函数理论模型,土壤有机碳和全氮的块金值/基台值的比值分别为1.8%和1.2%,有机碳和全氮的块金系数均小于25%,表明有机碳和全氮具有强烈的空间相关性。有机碳和全氮空间变异的尺度范围不同,分别为50.400 km和59.200 km。研究区的有机碳总体空间分布规律是有机碳在北部较高、南部较低,呈自北向南递减趋势,土壤全氮与有机碳的空间分布趋势相似,但有机碳的空间变异特征较全氮明显,这种空间分布格局主要受环境因子、 土壤质地、 土壤类型以及土地利用类型等的影响,其中环境因子中的气温和海拔对有机碳和全氮的影响较大。通过比较普通克里格和回归克里格的预测结果,回归克里格能较好地反映东南部有机碳和全氮较低地区的局部变异外,对于西北部的山区也能更好地反映碳、 氮与地形及气候等因素的关系。【结论】河北土壤有机碳和全氮的空间变异和分布特征较为类似,受地形地貌、 气候等因素的影响。通过比较普通克里格法和回归克里格法的空间预测结果,回归克里格法可以消除环境因子的影响,从而得到更准确的空间预测结果,因此建议使用回归克里格法进行预测,以期获得一个更为准确的土壤有机碳和全氮的空间预测结果。     

亚热带红壤丘陵典型区土壤全氮的空间变异特征

在GIS支持下,运用地统计学方法分析了耕层土壤全氮的空间变异特征,并在此基础上利用Kriging插值方法绘制了土壤全氮的空间分布图。结果表明,在步长间隔60 m下,土壤全氮具有较强的空间相关性,其相关距离为208 m。在NE30°、NE120°两个方向上具有典型的几何异向性结构特点;在NE60°、NE150°两个方向上具有典型的带状异向性结构特点。Kriging插值结果表明,研究区土壤全氮的空间分布表现为条带状和斑块状分布。土壤全氮高值斑块区的分布与地势相对低洼的地形部位相吻合。地形是影响土壤全氮空间变化的主要因素。     

施氮量和蚕豆/玉米间作对土壤无机氮时空分布的影响

在田间条件下于2006—2007年研究了不同氮水平下（N 0、75、150、225、300 kg/hm2）蚕豆/玉米间作体系与其相应单作体系土壤无机氮的时空分布规律,旨在为河西走廊灌区蚕豆/玉米间作体系的氮素管理提供理论依据。用土钻法采集土壤剖面样品,CaCl2浸提,流动分析仪测定土壤无机氮的方法研究了施氮量和蚕豆/玉米种间相互作用对土壤无机氮时间和空间变化特点。结果表明:灌漠土无机氮以NO3--N为主。蚕豆和玉米无机氮含量在蚕豆收获前种植方式间均无显著性差异,蚕豆收获后至玉米收获,间作显著降低了两种作物各层无机氮含量;无机氮含量随着施氮量增加而显著增加。蚕豆收获后间作体系0—100 cm土层无机氮累积量略高于单作体系,且0—100 cm 土层无机氮累积量高于100—160 cm土层;玉米收获后,间作蚕豆和玉米土壤无机氮累积量在0—100 cm土层分别平均降低了51.7%和16.6%,在100—160 cm土层平均降低了42.1%和6.1%;与不施氮相比,施氮蚕豆和玉米无机氮累积量在0—100 cm土层分别平均增加了40.1%和81.5%,在100—160 cm土层分别增加了69.6%和40.6%;与单作体系相比,间作体系0—100 和100—160 cm土层土壤无机氮分别降低43.4%和34.1%。因此,施氮肥显著增加土壤无机氮的累积,而豆科/禾本科间作减少了土壤无机氮的残留。