The effects of nutrient limitations on microbial respiration and organic matter decomposition in a Florida Spodosol as influenced by historical forest management practices

Recent investigations on the mechanistic underpinnings for nutrient regulated organic matter decomposition suggest that micronutrients may limit microbial activity in nitrogen (N) enriched forest ecosystems. However, these nutrient limitations could be complex for managed pine plantations in the US South that received macro- and micronutrient fertilization and weed control treatments. Soils and pine litter from second rotation loblolly pine stands were incubated to study the legacy effects of long-term silvicultural treatments [30+ years - Untreated Carryover (C) - Control (C-C), Fertilization (C-F), Fertilization + Weed control (C-FW), Weed control (C-W)] on microbial respiration and organic matter decomposition. These legacy treatment effects were contrasted with soils and litter corresponding to the current rotation's actively managed treatments that were the same as in the first rotation (Actively managed retreated-C, F, FW, W). In general, both past rotation (C-F: 17.9 mu g C g(-1) soil day(-1)) and current rotation fertilizer treatments (F: 22.3 mu g C g(-1) soil day(-1)) resulted in higher microbial respiration rates compared to their respective controls (C-C: 11.9 mu g C g(-1) soil day(-1); C: 12.5 mu g C g(-1) soil day(-1)), which likely reflected an inherently higher soil carbon content. Carbon normalized microbial respiration rates during the incubation period followed exponential decay patterns, with lower decay rates in fertilized soils compared to the average among treatments. Furthermore, N + phosphorus (P) additions suppressed microbial respiration in the C-F and F treatments, but accelerated it in the C-C and C treatments. This observation suggests that during the early stages of decomposition, N and P were limiting to microorganisms in those soils without a silvicultural treatment history. Positive microbial respiration response to added Cu for the C-C and C, and added Mn for the C-C, C, and W treatments suggested micronutrient limitations to microbial decomposition processes. For soils without a fertilization history, Mn peroxidase activity response to Mn addition levels followed the trend: high level > low level = No addition (C-C and C-W: p = 0.0032; C and W: p < 0.0001), and significantly correlated with microbial respiration rates (r = 0.63). Overall, the carryover effects from past forest management practices altered soil microbial and decomposition responses to nutrient additions, including alleviation of nutrient limitations to soil extracellular enzyme production in a Florida Spodosol.

Automated summary

Recent investigations suggest that micronutrients may limit microbial activity in nitrogen enriched forest ecosystems. Both past and current fertilizer treatments resulted in higher microbial respiration rates compared to controls, with nitrogen and phosphorus additions suppressing microbial respiration in some cases. This study highlights the complex interactions between nutrient availability and microbial activity in forest ecosystems.