Land use effects on C-N-and P stocks and greenhouse gas fluxes in agroecosystems in southern Chile

Agricultural and animal production are normally considered activities that degrade soils and are sources of greenhouse gases (GHG), such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Here, we present a detailed description of the carbon (C), nitrogen (N) and phosphorus (P) stocks in croplands (CR), grasslands (GR), native shrublands (NS) and invasive shrublands (IS) at three locations of northern Chiloé Island, southern Chile. Also, using a portable chamber system (1 m³), the three GHG fluxes were measured one day per month, for one year (2018). The results showed that the larger total stocks of C and N were found at the NS sites, with values of 50.5 ± 4.3 kg m^-2 and 2.96 ± 0.54 kg m^-2, respectively. In contrast, the larger stock of P was observed at the GR sites, with a value of 0.51 ± 0.08 kg m^-2. Comparisons of the total ecosystem stocks showed no significant differences among  the agroecosystems but differed from values reported for a forest in Chiloé, which revealed a significant loss in C (58.6%) and N (11.1%) stocks in the agroecosystems, while the P stock increased by 92% compared to the forest. As net sources of CO₂ acted the CR sites; net sources of CH₄ were the CR, GR and IS sites; and net sources of N₂O were the CR sites. The GHG balance showed that the CR sites behaved as a net source (388 g CO₂-eq m^-2 year^-1), while GR (-1248 g CO₂-eq m^-2 year^-1), NS (-1097 g CO₂-eq m^-2 year^-1) and IS (-1928 g CO₂-eq m^-2 year^-1) acted as sinks. This indicates that croplands could make an important contribution to local and regional GHG emissions. In a wider context, these results indicate that the regulation of land use conversions for agricultural use might be an effective tool to combat climate change, potentially reducing GHG emissions.