Differential effects of nutrient addition and woody plant encroachment on grassland soil, litter and plant dynamics across a precipitation gradient

Abstract

Interactions between woody plant encroachment and nutrient availability may vary along precipitation gradients. Furthermore, with predicted increases in N deposition due to increasing industrial emissions and changes in land use, potential increases in N storage and cycling are expected. We conducted a two-year field experiment that tested whether nitrogen (N) and phosphorus (P) addition and woody plant encroachment had independent effects on grasslands along a precipitation gradient. We added these nutrients to adjacent encroached and open grasslands (ranging from 300 to 1500 mm mean annual precipitation). We compared differential responses to fertilizer addition and encroachment in terms of soil N stocks and soil P, litter % N and C:N ratio (index of decomposition), plant productivity and species richness. In addition, we determined whether there were trends in responses to increased nutrient availability for encroached versus open grasslands across the above-mentioned precipitation gradient. Soil N stocks were on average 15 times higher in the high precipitation sites regardless of nutrient addition or encroachment status. Larger variation in soil P was noted at the high precipitation sites, with some evidence of P-limitation in the encroached area with the highest precipitation. Fertilization increased grass biomass and reduced species richness in the high precipitation sites only. Overall, increased nutrient availability was found to be of importance in high precipitation sites only, possibly due to water scarcity at the low precipitation sites. We found significant positive effects of encroachment on litter % N (quality), C:N ratios and forb cover across the precipitation gradient. The purportedly beneficial effects of encroachment on litter and plant species richness was more pronounced at the high-precipitation sites.