Nitrogen Balance in Intensively Managed Loblolly Pine Stands at Mid-Rotation
Abstract
Intensive management of pine plantations relies on constant manipulation of resources, where significant rates of nutrients, particularly nitrogen (N), are applied annually. However, we lack a complete understanding of N cycling in these forests, especially during early stages of stand development. A detailed accounting of N accrual in different forest pools through early stand development can improve our understanding of N-uptake, use and storage, and guide fertilization recommendations. We evaluated the effects of competition control, fertilization rates, and stand density on the N-balance of intensively managed Pinus taeda stands in southeastern US. Five treatments were tested: no herbicide and no fertilization (control - C); only herbicide (H); herbicide and half of N fertilization rate (HF); herbicide and full fertilization (FF); and increased stand density with herbicide and full fertilization (ID). N concentration ([N]) and N content (Nc) in soil, forest floor and living vegetation compartments (roots, stem, branches, needles, and competing vegetation) were quantified at mid-rotation (5-year stand). Regardless of treatment, the first meter of soil was the largest N pool, holding almost 4 Mg N ha-1. The soil upper 30 cm comprised ~ 40 % of the soil pool. Fertilization rate increased root-N; [N] and Nc content in roots followed the rank ID > FF > HF > H. Fertilization increased aboveground tree Nc, but the rate did not. Nc in aboveground tree biomass ranged from 9 - 130 kg N ha-1, with C and ID exhibiting the lowest and highest Nc, respectively. The competing vegetation in C exhibited 2-fold higher Nc than trees. Conversely, the Nc in aboveground tree biomass was more than 10-fold greater than in competing vegetation in all herbicide treatments. HF, FF & ID exhibited higher Nc in aboveground tree biomass than in the forest floor, whereas the opposite was observed for C & H. No decrease in soil N pool was observed with higher N uptake by trees. While herbicide was critical to N-uptake, the differences in aboveground tree biomass Nc between H, HF & FF suggest that fertilization is beneficial, but operational levels could be reduced without impacting forest production or nutritional status. However, the differences in root Nc might have critical consequences that need to be better investigated throughout the harvest rotation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B11J2241D
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES;
- 0495 Water/energy interactions;
- BIOGEOSCIENCES;
- 1836 Hydrological cycles and budgets;
- HYDROLOGY