Abstract:

Aims Land use change may alter nitrogen (N) dynamics in terrestrial ecosystems, but its effects remain poorly quantified. Due to reorganization of the land use by government, a large cultivated area has converted to woodland and shrubland plantation since the 1980’s in Danjiangkou Reservoir area, China. Our objective was to investigate effects of agricultural land use change on soil N dynamics in this area.
Methods We conducted in situ close-top tube incubation experiment to determine soil inorganic N (NH₄⁺-N, NO₃^--N), soil net nitrogen mineralization and nitrification in three adjacent land use types (afforestation, shrubland and cropland) in the Qingtang river basin around Danjiangkou Reservoir in Spring of 2011.
Important findings Soil NO₃^--N concentration (mg·kg^-1) was 1.27 ± 0.18, 3.51 ± 0.11 and 9.00 ± 0.73 in afforestation, shrubland and cropland, respectively. Soil NO₃^--N concentration significantly increased from afforestation to shrubland to cropland, whereas NH₄⁺-N concentration (mg·kg^-1) was not significantly different among afforestation (1.33 ± 0.20), shrubland (1.67 ± 0.17) and cropland (1.62 ± 0.13). We found that NO₃^--N was the main form of inorganic N under shrubland and cropland, and the proportion of NO₃^--N to soil inorganic N content was 67.8% and 84.8% in shrubland and cropland, respectively; however, the concentration of NO₃^--N was approximately equal to NH₄⁺-N under afforestation. Soil net N mineralization rate (mg·kg^-1·30 d^-1) were 0.98 ± 0.12, 2.52 ± 1.25 and 6.58 ± 2.29, respectively in afforestation, shrubland to cropland. Soil net N mineralization rate was not significantly different among the land use types, whereas nitrification rate (mg·kg^-1·30 d^-1) was significantly different: cropland (7.13 ± 2.19), shrubland (2.56 ± 1.07) and afforestation (0.85 ± 0.10). Soil net N mineralization rate was positively correlated with nitrification rate. Nitrification rate accounts for approximately 87% of soil mineralization rate in afforestation, whereas nitrification was higher than mineralization rate in shrubland and cropland soil due to the ammonification rate being below zero. Both soil N mineralization and nitrification were regulated by soil water content and soil temperature with more sensitive to soil water content during the incubation period. Additionally, soil N mineralization and nitrification were negatively correlated with soil C:N ratio. Our results suggest that change in soil microclimate and soil C:N ratio under land use change would impact soil N dynamics.

Key words: Danjiangkou Reservoir, land use type, net nitrogen mineralization rate, nitrification rate, soil inorganic nitrogen