Soil N dynamics in relation to leaf litter quality and soil fertility in north‐western Patagonian forests

Abstract

Summary We examined the relationships among soil N dynamics, soil chemistry and leaf litter quality in 28 forest stands dominated by conifers, woody broad‐leaf deciduous species or broad‐leaf evergreens. Potential net N mineralization, net nitrification and microbial biomass N were used as indicators of soil N dynamics; pH, organic C, total N, exchangeable cations and extractable P as indicators of soil chemistry and N concentration, lignin concentration, C : N ratio and lignin : N ratio in senescent leaves as indicators of leaf litter quality. N dynamics were assessed in two consecutive years with contrasting precipitation. Net N mineralization was lower in stands of the three conifers and one of three broad‐leaf evergreen species than in stands of the other six broad‐leaf species (40–77 vs. 87–250 mg N kg−1 after 16‐week incubations) and higher in the wetter year. The proportion of N nitrified was high beneath most species regardless of mineralization rates, soil N fertility and leaf litter quality, and was significantly higher for the wetter year. Ammonium was the predominant form of N in three sites affected by seasonal waterlogging and in two sites the predominant form changed from ammonium in the drier year to nitrate during the wetter year, probably due to differences in soil texture affecting soil moisture. Net N mineralization was linearly related to microbial biomass N, implying that the microbial activity per biomass unit was quite similar beneath all species. Constant microbial biomass during the wetter year suggested that as mineralization/nitrification increased, there was a higher potential risk of N losses. Although the litter lignin : N ratio allowed differentiation of soil N dynamics between broad‐leaf species and conifers, its constant value (23–28) in all broad‐leaf species made it a poor predictor of the differences found within this group. Across all sites and between broad‐leaf species, soil N dynamics were best explained by a combination of leaf litter lignin and soil chemistry indicators, particularly soil total N for net N mineralization and net nitrification, and soil organic C for microbial biomass N.