Nitrogen and phosphorus status in a ponderosa pine forest after 20 years of interval burning

We examined the cumulative effects of 20 years of two-year interval prescription burning on available nitrogen (N) and total N and phosphorus (P) pools in the upper 15 cm of mineral soil of a southwestern ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forest over a complete two-year burn cycle. Repeated burning reduced anaerobically mineralizable N up to 25%, However, burned and control plots had similar monthly ammonium and nitrate pool sizes in the mineral soil during both years of the two-year burn cycle. Mineral soil total N in the upper 15 cm was reduced by 110 kg N/ha (about a 7% reduction) after 20 years of prescription burning at this site. This estimate, coupled with a calculated 470 kg/ha loss of N from the forest floor, indicates repealed burning has substantially depleted soil N stocks at this site. In contrast to N, mineral soil total P in burned plots was similar to control plots. Our results suggest that repeated prescription burning at two-year intervals may have deleterious long-term effects on N cycling in southwestern ponderosa pine forests.