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. [ABSTRACT FROM AUTHOR]