Determining the harvest frequency to maintain grassland productivity and minimum nutrient removal from soil

Background Quantifying grassland aboveground biomass and soil nutrient removal in response to harvest intensities is essential to balance productivity and conservation benefit. There is a lack of studies examining the harvest frequency-induced threshold that leads to abrupt decay in grassland functioning. Methods Based on field experiments in a semiarid and a dry subhumid grassland, we tested threshold responses of aboveground biomass (AGB) and removed plant nitrogen (N) and phosphorus (P) to 9-harvest frequency gradients within-year (2017 and 2018); and the legacy effects of harvest on AGB and soil available N and P pools and stoichiometry following cessation (2019) of harvest. Results In both grasslands, the plant AGB within-year were highest at the frequency of ~2-harvest; it abruptly decreased once the threshold was exceeded. Moreover, except for plant N at the semiarid site, the response patterns of the N and P removals to harvest frequency were similar to that of plant AGB. The increased removal of N was higher than P with harvest frequency, leading to decreased soil available N/P ratio excess of twice at the dry subhumid site. Following cessation, the previous harvest frequency had a negative legacy effect on both plant AGB and the ratio of graminoid to forb species. Conclusions These results indicate that ≤2-harvest annually are applicable for optimal pastoral production; among which one harvest by the end of the growing season may be appropriate for conserving above- and below-ground parts and maintaining healthy grassland functions. Soil N/P condition should be considered as an indicator when determining a sustainable harvest frequency.