Slash-and-burn in karst regions lowers soil gross nitrogen (N) transformation rates and N-turnover

Abstract

Slash-and-burn is a common practice in the subtropical karst areas of southwest China, as well as in other places around the world, to prepare forest land for agricultural plantations. Understanding the effect of slash-and-burn practice on soil nitrogen (N) supply capacity is critical for the preservation of ecosystem services including long-term plant productivity. This is of particular relevance in mountainous karst regions with thin calcareous soils characterized by high pH, high calcium (Ca), and high soil loss risk. In this study, a 15N tracing approach was employed to quantify the effect of slash-and-burn on gross N-transformation rates and the mineralization-immobilization turnover (MIT) in soils from subtropical karst regions half-year after the burning, right before the agricultural cultivation began. Soil samples were collected from burned and nearby control sites (woodland). Compared to the control sites, the contents of soil organic carbon, total N, and Ca were lower in the burned sites, while no significant difference was observed in soil enzyme activities. The rates of mineralization of organic N to ammonium (NH4+), NH4+ adsorption and release, NH4+ oxidation to nitrate (NO3–) (i.e., autotrophic nitrification, ONH4), and microbial NO3– immobilization were significantly lower in burned sites, suggesting that slash-and-burn has a negative effect on MIT in karst regions. The mean residence times of inorganic N were 4.33 ± 0.53 d-1 for NH4+ and 6.83 ± 1.76 d-1 for NO3– in burned soil, which is remarkably higher than the control soil (0.77 ± 0.08 d-1 for NH4+ and 2.47 ± 0.83 d-1 for NO3–), indicating decreased N turnover after slash-and-burn. Abundances of bacteria and fungi, reflected by 16 s rRNA and ITS gene analyses, were not different between burned and control sites, suggesting that microbial community composition and activity, rather than the population size, controls the microbial-modulated N transformations. Owning to the slower NH4+ and NO3– turnover and reduced soil N transformation rates, we conclude slash-and-burn practice significantly lowers soil inorganic N supply capacity, leading to remarkably reduced soil inorganic N content, especially for NO3– (from 59.9 ± 8.67 mg N kg−1 to 44.1 ± 1.86 mg N kg−1), which may further restrict the efficiency of agricultural exploitation. Thus, we recommend that the follow-up agricultural activities after slash-and-burn practice need to take this into account in addition to erosion control and nutrient conservation measures.