Nitrogen Cycle Patterns During Forest Regrowth in an African Miombo woodland Landscape

Tropical dry forests in eastern and southern Africa cover 2.5 x 10(6) km(2), support wildlife habitat and livelihoods of more than 150 million people, and face threats from land use and climate change. To inform conservation, we need better understanding of ecosystem processes like nutrient cycling that regulate forest productivity and biomass accumulation. Here we report on patterns in nitrogen (N) cycling across a 100-year forest regrowth chronosequence in the Tanzanian Miombo woodlands. Soil and vegetation indicators showed that low ecosystem N availability for trees persisted across young to mature forests. Ammonium dominated soil mineral N pools from 0- to 15-cm depth. Laboratory-measured soil N mineralization rates across 3- to 40-year regrowth sites showed no significant trends and were lower than mature forest rates. Aboveground tree N pools increased at 6 to 7 kg Nha(-1)yr(-1), accounting for the majority of ecosystem N accumulation. Foliar delta N-15 <0 in an N-fixing canopy tree across all sites suggested that N fixation may contribute to ecosystem N cycle recovery. These results contrast N cycling in wetter tropical and Neotropical dry forests, where indicators of N scarcity diminish after several decades of regrowth. Our findings suggest that minimizing woody biomass removal, litter layer, and topsoil disturbance may be important to promote N cycle recovery and natural regeneration in Miombo woodlands. Higher rates of N mineralization in the wet season indicated a potential that climate change-altered rainfall leading to extended dry periods may lower N availability through soil moisture-dependent N mineralization pathways, particularly for mature forests. Plain Language Summary In dry tropical Miombo woodlands of sub-Saharan Africa, forests have experienced high rates of clearing and degradation from growing demands for land and resources (e.g., charcoal, firewood). A lack of knowledge regarding controls on forest regrowth, such as nutrient cycling, has limited development of recommendations for managing regrowth as part of forest landscape restoration. This work studied patterns of ecosystem nitrogen (N) availability with regrowth, and how the sensitivity of N availability from an important soil decomposition process (N mineralization) varied between wet and dry seasons in the Tanzanian Miombo. Across 12 sites spanning a 100-year regrowth chronosequence, increases in ecosystem N stocks were dominated by aboveground vegetation (trees). Soil N mineralization of regrowth sites did not recoup rates of mature forests. Sources and processes whereby trees accumulated N were unclear from the data, but evidence suggested the importance of redistribution of N from soil to vegetation and a possible role for N fixation. We also found in lab measurements that N mineralization rates were 2 to 3 times higher in the wet than dry season. For management of regrowth and forest landscape restoration, results indicated the importance of maintaining aboveground biomass in regrowth forests for conserving ecosystem N resources.