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.

土地使用方式的变化会导致各种元素（尤其是氮）的循环发生重大变化，而氮容易受到人为干扰。为了量化这些变化，需要精确测量N的生物地球化学转化速率。我们使用（¹⁵ N）同位素稀释技术，通过比较管理森林（一种非管理森林）的土壤中的总氮矿化率和铵态氮（NH ₄ ⁺）消耗率，来了解森林改变类型对（N）转化率的影响，以及印度喀拉拉邦的橡胶园。总体而言，硝酸盐（NO ₃ ^- ）的管理和非管理森林的主导的土壤，而在橡胶种植土壤表现出相对较高的NH ₄⁺浓度。橡胶冠层下的总N（TN）和总有机碳（TOC）浓度最高（TN：1.49±0.02 mg N g ^-1； TOC：7.96±0.86 mg C g ^-1）。在所有三种森林类型的土壤中，总的氮矿化率高于NH ₄ ^+的消耗率。尽管TN和TOC浓度较高，但橡胶种植园中的总N矿化率和NH ₄ ⁺消耗率要低得多（矿化度：1.08±0.08 mg N kg ^-1 d ^-1 ;消耗量：0.85±0.09 mg N kg ^-1 d ^- ）相比，管理（矿化：3.71±0.35毫克N-千克^-1d ^-1 ; 消耗量：2.20±1.41 mg N kg ^-1 d ^-1）和未管理（矿化：2.20±1.07 mg N kg ^-1 d ^-1；消耗量：1.39±0.27 mg N kg ^-1 d ^-1）的森林。橡胶园中较低的NH ₄ ⁺消耗速率导致 NH ₄ ^+的停留时间显着增加（p <0.05）。（〜4天）与未管理和未管理的森林（<2天）相比，可能导致橡胶土壤酸化（pH〜4.8）。这些结果共同表明，用单作种植的人工林替代自然生长的森林（例如橡胶）会对热带土壤中氮的转化过程产生负面影响，并暗示自然生长的森林的树种组成变化会对土壤微生物活性产生不利影响。我们建议在这些人工林和商品作物之间套种，以维持土壤微生物多样性和生物地球化学循环，以实现可持续森林管理。