Flow and fate of soil nitrogen in an annual grassland and a young mixed-conifer forest

Abstract

A comparative study of N dynamics in an annual grassland and a young mixed-conifer forest, that occur on the same soil great group in California, was made by adding small amounts of 15NH 4 + to mineral soils within field microplots and following changes in 15N recoveries in soil and plant pools over 12–16 months. In annual grassland microplots, the aboveground plant biomass contained 5–6% of the added 15N at the end of the growing season. One month after 15N addition almost 70% of the added 15N was recovered in the soil organic matter (SOM) pool, which included surface litter and roots but not microbial biomass. Laboratory incubation of sterilized soil cores indicated that as much as half of the 15N recovered in the SOM pool may have been fixed abiotically. Nevertheless, substantial and rapid incorporation of 15N in the SOM pool indicates a high potential for N immobilization and a rapid turnover of the microbial biomass-N pool in this soil. Recovery of 15N in microbial biomass ranged from 9 to 15%; recovery in the soil solution was lower than in the microbial biomass, but showed a similar seasonal trend. Total 15N recovery ranged from 72 to 85% over the study period. In forest microplots (which excluded plants), recovery of added 15N was generally lower than in the grassland for all soil pools examined. Recovery of 15N in SOM and microbial pools showed reciprocal trends seasonally, suggesting that there was considerable movement of N between these pools in the forest soil. Total 15N recovery was only about 54% 6 months after 15N addition, indicating a high potential for rapid N loss from the surface soil of this forest. Substantially lower recovery of 15N in the forest soil cannot be attributed entirely to the exclusion of plant uptake because plant 15N recovery in the grassland was relatively low. The low 15N recovery in the forest soil might be due to the low capacity of this soil to immobilize N and protect immobilized N from being remineralized and lost from the soil via leaching or denitrification. In both ecosystems estimates of N flow from the mineral soil to decomposing surface residues (presumably via fungal translocation) was significant (0.29 and 0.02 g-N m −2 yr −1 for the grassland and forest, respectively) relative to the net internal dynamics of N within the surface detritus. Comparison of net changes in litter N and accumulation of 15N during decomposition suggests that N was mineralized from litter concurrently as N was immobilized from the mineral soil in both sites.