Effects of the type of forest alteration on gross nitrogen mineralization in soils of southern India

Abstract

Change in land-use practices can result in major shifts in the cycling of various elements, particularly nitrogen (N), which is prone to anthropogenic perturbations. For quantifying these shifts, accurate measurements of rates of biogeochemical transformations of N are needed. We used the (15N) isotope dilution technique to understand the effects of the types of forest alteration on (N) transformation rates by comparing gross N mineralization and ammonium (NH4+) consumption rates in soils of a managed forest, an unmanaged forest, and a rubber plantation in Kerala, India. Overall, nitrate (NO3−) dominated soils of the managed and unmanaged forests, whereas soils in the rubber plantation showed relatively higher NH4+ concentration. Total N (TN) and total organic carbon (TOC) concentrations were the highest under the rubber canopy (TN: 1.49 ± 0.02 mg N g−1; TOC: 7.96 ± 0.86 mg C g−1). In soils of all three forest types, gross N mineralization rates were higher compared to NH4+ consumption rates. Despite high TN and TOC concentrations, the rates of gross N mineralization and NH4+ consumption were considerably lower in the rubber plantation (mineralization: 1.08 ± 0.08 mg N kg−1 d−1; consumption: 0.85 ± 0.09 mg N kg−1 d−) compared to the managed (mineralization: 3.71 ± 0.35 mg N kg−1 d−1; consumption: 2.20 ± 1.41 mg N kg−1 d−1) and unmanaged (mineralization: 2.20 ± 1.07 mg N kg−1 d−1; consumption: 1.39 ± 0.27 mg N kg−1 d−1) forests. The lower NH4+ consumption rates in the rubber plantation led to significantly higher (p < 0.05) residence time of NH4+ (~ 4 days) compared to the managed and unmanaged forests (< 2 days), possibly contributing to acidification of rubber soils (pH ~ 4.8). These results together suggest that replacement of naturally grown forests with a mono-cropped plantation such as rubber negatively impact rates of N transformation processes in tropical soils and imply that change in tree species composition of naturally grown forests can adversely affect soil microbial activity. We recommend intercropping these plantations with commercial crops to maintain soil microbial diversity and biogeochemical cycling for sustainable forest management.