Bioturbation by mammals and fire interact to alter ecosystem-level nutrient dynamics in longleaf pine forests

Activities of ecosystem engineers can interact with other disturbances to modulate rates of key processes such as productivity and nutrient cycling. Bioturbation, movement of soil by organisms, is a widespread form of ecosystem engineering in terrestrial ecosystems. We propose that bioturbation by southeastern pocket gophers (Geomys pinetis), an abundant but declining ecosystem engineer in longleaf pine (Pinus palustris Mill.) forests, accelerates nutrient dynamics of the forest floor by burying litter and then reduces litter consumption and nitrogen (N) volatilization losses in the presence of fire. We evaluated our hypothesis by measuring how litter burial alters decomposition and N and phosphorus (P) turnover of longleaf pine and turkey oak (Quercus laevis Walt.) litter over four years, and then simulated interactive ecosystem-level effects of litter burial and low-intensity fires on N and P dynamics of the litter layer. In the field, mass loss was over two times greater and N and P were released much more rapidly from litter buried beneath mounds than on the surface of the forest floor. At a measured rate of mound formation covering 2.3 ± 0.6% of the forest floor per year, litter mass and N and P content of the forest floor simulated over an eight-year period were approximately 11% less than amounts in areas without pocket gopher mounds. In contrast to unburied litter, litter beneath mounds is protected from consumption during fires, and as fire interval increased, consumption rates decreased because mounds cover more years of accumulated litter. Our research indicates that bioturbation and burial of litter by pocket gophers accelerates turnover of N and P on the forest floor, and in the presence of fire, conserves N in this ecosystem where productivity is known to be nutrient limited.

生态系统工程师（ecosystem engineers）的活动可与其他干扰因素相互作用，调控生产力、养分循环等关键生态过程的速率。生物扰动（bioturbation）即生物对土壤的搬运活动，是陆地生态系统中广泛分布的一类生态系统工程形式。我们提出，东南部囊鼠（southeastern pocket gophers，*Geomys pinetis*）——长叶松（longleaf pine，*Pinus palustris* Mill.）林中一类数量丰富却正逐步衰退的生态系统工程师——的生物扰动，可通过掩埋凋落物加速林地表层的养分循环，并在火干扰存在的情况下减少凋落物消耗与氮（N）挥发损失。为验证该假说，我们开展了为期四年的野外实验，测定凋落物掩埋如何改变长叶松与火鸡栎（turkey oak，*Quercus laevis* Walt.）凋落物的分解过程以及氮、磷（P）周转速率；随后模拟了凋落物掩埋与低强度火烧共同作用下，凋落物层氮、磷动态的生态系统级响应。野外监测结果显示，丘冢下方掩埋的凋落物其质量损失率是林地表层裸露凋落物的两倍以上，氮与磷的释放速率也显著更快。据测定，每年丘冢形成覆盖的林地表面积占比为2.3±0.6%，基于此参数模拟的八年周期内，林地表层的凋落物总质量以及氮、磷储量较无囊鼠丘冢的区域低约11%。与未掩埋的裸露凋落物不同，丘冢下方的凋落物在火烧过程中可得到保护；且随着火烧间隔期延长，凋落物消耗率会进一步降低，因丘冢覆盖了多年积累的凋落物。本研究表明，囊鼠的生物扰动与凋落物掩埋作用可加速林地表层氮、磷周转，且在火干扰存在时，能在这一生产力受养分限制的生态系统中实现氮素的固持。