Legume versus non-legume foliar litter decomposition in regularly burned loblolly pine forests

Abstract

While understory prescribed burning is central to the ecological and economic management of many pine forests, there is concern that burning may cause excessive losses of N, thereby decreasing soil fertility and forest productivity. Symbiotic N2-fixation by fire-adapted native and naturalized herbaceous legumes may mitigate N losses due to burning. However, the N gain via legumes depends not only on the rates of N2-fixation, but also on the rates of litter decay between burn events. At present, the rates and controls of relatively N-rich legume litter decay in forests are poorly understood. In this study, foliar litter decomposition was assessed for 6 legume species (average [N] = 1.83 ± 0.52 %) and 4 non-legume species (average [N] = 0.80 ± 0.27 %) in “early” and “late” stage loblolly pine (Pinus taeda) forests for 3 years. We hypothesized that the N-rich legumes would exhibit greater mass loss, particularly in the early sites, and would thus release a greater percentage of their original N compared to the non-legumes. In contrast to our hypothesis, average mass loss rates were comparable for the legumes and non-legumes resulting in similar residual mass estimates (mean [95 % CI]) respectively in the early (11.9 % [9.8 %, 14.4 %] versus 15.5 % [12.2 %, 19.7 %]) and late (12.5 % [10.3 %, 15.1 %] versus 9.0 % [7.1 %, 11.3 %]) sites by the end of the study. Likewise, the average decay rate (k) for the legumes and non-legumes did not differ significantly in the early sites (0.73 ± 0.25 year−1 versus 0.69 ± 0.15 year−1, respectively), and the average legume decay rate was lower (P = 0.0080) than the average non-legume decay rate in the late sites (0.58 ± 0.18 year−1 versus 0.63 ± 0.13 year−1, respectively). Mass loss was typically greater in the early sites, but the differences were significant for less than 22 % of the comparisons. As hypothesized, legumes did release a greater percentage of their original N compared to non-legumes resulting in lower average N remaining estimates respectively in the early (34.2 % [29.4 %, 39.8 %] versus 51.3 % [40.2 %, 65.5 %]) and late (28.9 % [24.2 %, 34.5 %] versus 42.5 % [29.6 %, 61.1 %]) sites after 3 years. The results suggest that N, P, and S were co-regulators of litter decay, and that pine litter may be an important sink for these limiting macronutrients. The strong interplay among N, P, and S should be considered in the use of prescribed burning to reduce nutrient limitations in fire-maintained pine forests.