Drought conditions alter litter decomposition and nutrient release of litter types in an agroforestry system of China

Drought changes litter quantity and quality, and soil microbial activities to affect soil nutrients in moso bamboo forest

Drought would significantly influence the forest soils through changing the litterfall production and decomposition process. However, comprehensive in situ studies on drought effects in subtropical forests, especially in bamboo forests, have rarely been conducted. Here, we conducted a throughfall exclusion experiment with a rainfall reduction of ~80% in moso bamboo (Phyllostachys edulis) forests to investigate effects of drought on litter quantity, quality, soil microbial and enzyme activities, and soil nutrients across two years in subtropical China. We observed that throughfall exclusion (TE) treatment significantly decreased soil moisture by 63% compared to ambient control treatment (CK). Drought significantly decreased the annual litterfall in the second treatment year, and the leaf litter decomposition rate (-30% relative to CK) over 2 years of decomposition. TE treatment significantly decreased net release rate of litter carbon (C) and the amount of litter nitrogen (N) immobilization during a 360-day decomposition period, leading an increased litter C: N ratio in TE compared to CK. There was a distinct difference in soil microbial community composition between TE and CK treatments, showing higher bacteria biomass in TE but no difference in fungal biomass between TE and CK. Structural equation modelling revealed that drought decreased the contribution of litter quantity to soil nutrients but increased that of litter quality and soil microbial community to soil nutrients. Our results suggest that increasing drought events in subtropical China will directly reduce litterfall quantity and quality on the one hand, and alter the soil enzyme activities and microbial composition on the other hand, all of which will consequently decrease litter decomposition rate, soil nutrient availability, growth rate and productivity, leading to changes in the functioning and services of subtropical bamboo forests.

Litter nitrogen concentration changes mediate effects of drought and plant species richness on litter decomposition

Biodiversity loss, exotic plant invasion and climatic change are three important global changes that can affect litter decomposition. These effects may be interactive and these global changes thus need to be considered simultaneously. Here, we assembled herbaceous plant communities with five species richness levels (1, 2, 4, 8 or 16) and subjected them to a drought treatment (no, moderate or intensive drought) that was factorially combined with an invasion treatment (presence or absence of the non-native Symphyotrichum subulatum). We collected litter of these plant communities and let it decompose for 9 months in the plant communities from which it originated. Drought decreased litter decomposition, while invasion by S. subulatum had little impact. Increasing species richness decreased litter decomposition except under intensive drought. A structural equation model showed that drought and species richness affected litter decomposition indirectly through changes in litter nitrogen concentration rather than by altering quantity and diversity of soil meso-fauna or soil physico-chemical properties. The slowed litter decomposition under high species diversity originated from a sampling effect, specifically from low litter nitrogen concentrations in the two dominant species. We conclude that effects on litter decomposition rates that are mediated by changing concentrations of the limiting nutrient in litter need to be considered when predicting effects of global changes such as plant diversity loss.