Spatial Patterns Of Soil Nutrients And Their Response To Grazing Disturbance Over The Songnen Meadow Steppes Of NE China

Soil is a continuum with high spatio-temporal heterogeneity which might be defined as spatial patterns as well. Ecological disturbance is one of the most important causes forming the spatial heterogeneity. Therefore, it is important to establish the dynamics of soil nutrients spatial patterns under grazing disturbance in grassland ecosystems; and it is also meaningful theoretically for recognizing the relations between grazing pressures and soil spatial heterogeneity; else, it is also helpful to further understand the interactions between grazing and soil nutrients patterns in grasslands ecosystem.Our research was conducted focusing on the grassland soil nutrients'spatial patterns under different grazing pressures on the Songnen meadow steppes. By use of site investigation and laboratory analysis, soil nutrients'contents, spatial variability, and correlation of different nutrient components under diverse grazing gradients were studied with the aid of statistic and geo-statistical methods. Data analysis was applied due to the semivariogram method and Kriging algorithm. Soil nutrients'spatial patterns under different grazing pressures were quantified and compared mutually. Herein, the grazing gradients were defined as naturally ungrazed meadows, lightly grazed meadows, moderately grazed meadows, and heavily grazed meadows. The soil nutrient indicators were selected as SOM, soil total nitrogen, total phosphorus, total potassium and soil available nitrogen, available phosphorus, and available potassium.Finally, some conclusions were established as follows:1.The coefficients of variability (CV) of all the target nutrients indicators appear all in the range of 7.6～29.1%, in which the CV of available phosphorus is the biggest while that of available nitrogen is at the down end. Soil organic matter (SOM), total nitrogen, available phosphorus perform with medium variance, while that of total phosphorus, available nitrogen, available potassium do with weak variance. The correlation analysis of each soil nutrient components shows that, the SOM performs extremely significantly positive with total phosphorus and significantly positive with available phosphorus.2.In natural grasslands, soil nutrients concentrate in the upper layer of the profile and decrease along the profile from top to lower. The distributions of SOM, total nitrogen, total phosphorus decrease gradually along the soil depth. Available nitrogen in 0～50㎝ layer, and available phosphorus in 0～100㎝ layer distribute with a"S"shape along the soil profile. Vertically, these soil nutrient variables vary with obvious stratification and surface accumulation features.3.Heavily intense grazing might result in the decrease of SOM in 0～10㎝ layer, while light grazing and moderate grazing may have an opposite impact. Soil total nitrogen content increase positively with the increasing grazing pressure. The content of soil total phosphorus showed highest at 10～50cm layer in heavily-grazed grasslands. However, soil total phosphorus showed the highest in 0～10㎝ layer of the moderately-grazed grassland. Overall, SOM, soil total nitrogen and total phosphorus showed similar spatial patterns under different grazing pressures.4. The available nitrogen under grazing showed higher than that of non-grazed ones with a limited variation range along the soil profile. With the increasing of grazing pressure, the content of soil available phosphorus increased at 0～10㎝ soil layer, but decreased swiftly at 10～20㎝ in heavily-grazed grasslands. The contents of available potassium in heavily-grazed grasslands revealed higher than that of the other grasslands; and the available potassium of ungrazed grasslands showed the highest at 20～30㎝ layer and 30～50㎝ layer.5.The semi-variograms of SOM, soil total phosphorus and available phosphorus were best described by exponential model; the best model for semi-variogram of soil total nitrogen, available nitrogen and available potassium were spherical model. Among the six types of target soil nutrient components, SOM, total nitrogen and total phosphorus have a moderate spatial dependence, while available nitrogen, available phosphorus and available potassium, have very weak spatial dependence.