Assessing the role of native herbaceous legumes in the nitrogen cycle of regularly burned loblolly pine forests

Abstract

Prescribed burning is one of the most commonly used management practices in pine (Pinus spp.) forests of the southeastern United States. There is concern, however, that burning may lead to excessive losses of nitrogen (N), thereby decreasing forest productivity. The N losses due to burning may be potentially countered via symbiotic N2-fixation by native and naturalized herbaceous legumes that may thrive in burned forests. However, the rates and controls of N input via legume N2-fixation in these ecosystems are poorly understood. The objectives of this study were to measure legume abundance, biomass, N2-fixation, substrate quality, and associated environmental controls in regularly burned loblolly pine (P. taeda) forests. The study sites were located in four management blocks at the Piedmont National Wildlife Refuge. This assessment revealed a total of 23 legume species in 9 genera. Legume abundance averaged 12.95 ± 2.30 individuals m−2, and collectively Centrosema virginianum, Chamaecrista nictitans, Desmodium marilandicum, Galactia volubilis, and Lespedeza procumbens constituted 70.8% of the total legume abundance. Legumes comprised 17.1% of the understory aboveground biomass, and legume foliage, stem, and total (foliage + stem) biomass estimates averaged 15.38 ± 3.18, 20.88 ± 4.01, 36.26 ± 6.95 g m−2, respectively. Legume foliar N concentration averaged 2.22 ± 0.06% (ranging from 1.42−2.83%), and all legume foliar N concentrations were significantly higher than the average foliar values for blackberry (Rubus spp., 1.10 ± 0.05%) and the aggregate of non-leguminous forbs (1.03 ± 0.08%) that grew concurrently with the legumes in the study sites. N2-fixation assessments, based on the differential between legume and non-N2-fixing reference plant foliar N concentrations, indicated that approximately 50% of legume N was derived from the atmosphere. Legume foliage tissues exhibited relatively high substrate quality indices such as high N concentrations, low lignin concentrations (average of 14.3 ± 1.9%), and correspondingly low lignin:N ratios (average of 6.7 ± 0.9) indicative of tissues that exhibit rapid mass loss and N mineralization rates during decomposition. Canopy openness was the environmental variable that was most strongly correlated with foliar N and areal biomass among the dominant legume species. The estimated N input of 0.14 g m−2 yr−1 via N2-fixation may compensate for N losses due to relatively low intensity and infrequent understory burns during a stand rotation. Additional studies are needed to more comprehensively assess legume N2-fixation inputs, associated environmental constraints, and practical field applications which may increase N input via symbiotic N2-fixation in regularly burned pine forest ecosystems.