Spatiotemporal heterogeneity of soil nitrogen mineralization in a Picea stand and its relation to soil physicochemical factors

In a typical natural secondary Picea stand which is located in Guandi Mountain, Shanxi Province, North China, 920 soil cores from 0 to 10 cm layer and 10 to 20 cm layer over 46 spatial locations (sampled by geostatistic principles and at different distances of 1 m at the minimum and 28.3 m at the maximum) are collected at regular intervals during five consecutive months by using a top-covered PVC tube and subject to in-situ incubation. By measuring soil physicochemical factors such as NO3−-N, NH4+-N, water content, bulk density, total nitrogen, organic matter and pH value, this study analyzes horizontal and vertical spatial heterogeneities and temporal and seasonal variations of soil N mineralization in the Picea stand and their relations to soil physicochemical factors. The results show that in both 0–10 cm and 10–20 cm layers, the peak value of NH4+-N content appears in June and tends to decline thereafter to the lowest point in October. NO3−-N content maintains a growth trend month by month during the experiment. NH4+-N and NO3–N content in 0–10 cm layer is significantly higher than that in 10–20 cm layer, and there is a heterogeneous horizontal spatial distribution at both layers. Net nitrification rate in 0–10 cm layer reaches the peak in July (0.285 mg kg d−1), but declines in August (0.177 mg kg d−1) and falls to the bottom in September (0.034 mg kg d−1). No significant monthly variation is found in net nitrification rate of 10–20 cm soil layer. The highest net N mineralization rate of both layers appears in May (0.276 and 0.212 mg kg d−1), then decreases continuously to a negative value in September. N mineralization in 0–10 cm layer is significantly higher than that in 10–20 cm layer. Both net nitrification and N mineralization rate are heterogeneously distributed in horizontal in both layers in each month, their coefficients of variation are over 72.26% and 92.58%, respectively, in which the spatial structure variance accounts for over 63.5% and 50.1% of total variance respectively, while they are both significantly auto-correlated at a smaller scale. Spatiotemporal heterogeneity of soil nitrogen mineralization in the Picea stand is remarkably correlated to soil physicochemical factors.