Effect of grazing on seasonal patterns of plant biomass, plant nitrogen and soil nutrients in California’s annual grasslands

Background/Question/Methods

California’s grassland ecosystems experience strong seasonality of both temperature and rainfall, which results in temporal variability in plant production and soil nutrients. Livestock grazing can accentuate or dampen these seasonal patterns in soil nutrient availability, which in turn, can govern the extent of plant regrowth in the current growing season, as well as in subsequent years (through changes in seed production). Knowledge of how grazing affects the seasonality of nutrient availability in California’s grasslands is limited, and may account for the variable effects of grazing on nutrient availability - with previous studies finding increases, decreases or no change. To assess the impact of grazing on the seasonality of plant biomass, plant nitrogen and soil nutrients, we measured above- and belowground biomass, plant nitrogen (N) content and availability of soil nutrients including total inorganic N, ammonium (NH₄), nitrate (NO₃), iron (Fe), phosphorus (P) and potassium (K) (determined by resin probes) at 4 times during the 2012-2013 growing season. We monitored these variables in 6 grazed pastures and compared them to ungrazed exclosures, across a mix of continuously and seasonally grazed pastures at UC Sierra Foothill and Extension Center in Browns Valley, CA.

Results/Conclusions

Seasonal trends were similar in grazed and ungrazed treatments, for all variables measured. While grazing did not have strong effects on seasonality of nutrients, it did impact total availability. Grazing increased soil inorganic N and Fe, but decreased K. While grazing is expected to decrease standing aboveground biomass, it also reduced belowground biomass. Higher soil N-availability in grazed plots may decrease the need for high root allocation in wetter years, but low root production can limit access to soil moisture. Grazing increased seed production in 2012, but decreased it in 2013. Year effects may be attributed to climactic variation, with 2012 experiencing a relatively wet spring while 2013 experienced dryer conditions. These changes in seed production have the potential to impact plant composition, density and production in the following growing season, as well as granivore populations.