The University of Georgia (UGA), states that the primary objective of the work was to investigate the importance of biological nitrogen (N) fixation in longleaf pine ecosystems influenced by land management. UGA expected that the soil-based process of N fixation would be central to recovery from disturbance in longleaf pine forests because they are inherently poor in N and disturbances (e.g., fire and military training) cause them to become even more N poor (Boring et al. 2004, Maloney et al. 2008, Nave et al. 2011). Biological fixation of atmospheric N2 by soil microbes (so-called “N-fixers”) offers a unique mechanism capable of rehabilitating the natural N cycle (e.g., Hendricks and Boring 1999, Hainds et al. 1999), but very little is known about these organisms and their activities in longleaf pine ecosystems.

In this project, UGA addressed three objectives in Resource Conservation Statement of Need (RCSON)-13-02 relating to soil functional diversity, the role of soil organisms in facilitating recovery from disturbances and ecosystem processes that regulate soil communities in the context of longleaf pine savannas. The motivation from UGA's works comes from previous Strategic Environmental Research and Development Program (SERDP) funded projects that suggested that disturbances (e.g., fire at too high a frequency and/or military training) could lead to N limitation and unsustainable management of longleaf pine (Garten and Ashwood 2004, Garten 2006). Here, UGA sought to fill this knowledge gap by examining the ecology of N fixation in these ecosystems, including the patterns of fixation associated with fire and a gradient of longleaf pine stand age.

The primary objectives of its research were to:

Reduce uncertainties in the understanding of N fixation in longleaf pine ecosystems including:

1) The relative contribution of various N fixing organisms (legumes, biological crusts, and free-living bacteria) in space and time; and

2) The interactions of soil nutrients (N, P, and Mo) as constraints for the various types of N fixation and how stand age and fire influence the abundance and activity of N-fixers directly (disturbance) and indirectly (through nutrient cycling processes);

Quantify the importance of N fixation at the ecosystem level, by determining:

3) The role of fixation as a source of N relative to other ecosystem fluxes and pools; and

4) Whether the mechanism of N fixation can counteract losses via fire volatilization.

UGA presented 5 main hypotheses that were tested in the context of these objectives:

1) N-fixing organisms vary in the amount and timing of their N inputs to longleaf pine ecosystems. N fixation rates are: a) slow but consistent over space and time (free-living fixers in litter layer); b) moderate and patchy over space, consistent over time (biological crusts); and c) rapid and patchy over space and time (herbaceous legumes).

2) Abundant available nitrogen generally acts to suppress N fixation, but the soil nutrient most limiting N fixation will vary among taxonomic groups such that: Mo limits free-living N-fixers in organic soils and P limits legumes.

3) Disturbances have both direct and indirect effects on N-fixing organisms, where fire and stand age affect the biomass of N fixing organisms (legumes and crusts) or habitat (free-living N-fixers) and indirect effects can manifest as interactions with soil nutrient availability.

4) N fixation is a critical source of N for sustaining the productivity of longleaf pine ecosystems.

5) The potential for N inputs from N fixation to balance losses in longleaf pine ecosystems relates to the magnitude and frequency of forest disturbances (e.g., fire and stand age) and the interacting effects of soil nutrients to sustain N fixation during periods of recovery.

UGA’s work was conducted at Fort Benning Military Installation and Eglin Air Force base, which its states are important for meeting the military training mission and contributing to regional conservation in the southeastern U.S. UGA states that it matched sites based on their soil texture in sandhill communites (turkey oak, pyrogenic grasses and longleaf) since they are common on military bases throughout the Southeast (Benning, Gordon, Moody, Pope, Eglin and Stewart all have substantive portions of their base dominated by sandhills). An additional reason for focusing on sandhill communities is that fire and disturbance trigger N limitation more rapidly in these coarse textured soils (Garten and Ashwood 2004) and impact the presence of legumes (Hiers et al. 2007). The motivation from its works comes from previous SERDP funded projects that suggested that disturbances (e.g., fire at too high a frequency and/or military training) could lead to N limitation and unsustainable management of longleaf pine (Garten and Ashwood 2004, Garten 2006).

Across 54 1-ha plots of longleaf pine at Fort Benning and Eglin Air Force Base representing a 227 year gradient of stand recovery, UGA quantified N losses from fire, patterns of N demand and availability, and quantified N fixation by legumes, soil crusts, and asymbiotic bacteria.

Three findings provide support for NGA’s “excess N hypothesis.” First, NGA found that exceptionally high N mineralization rates (12.8 to 92.3 kg N ha-1 year-1) characterize its youngest stands, suggesting that these stands are replete with N following transitions from prior land use. Second, N demand from tree growth remains steady over stand age suggesting that forest productivity is not declining over time, as would be expected if N loss was leading to N deficiency. Third, N addition from NGA’s field fertilization experiment did not increase the growth rates of trees, even in older stands, providing additional evidence of N sufficiency. However, without historical (i.e., pre-European settlement) data on longleaf pine N stocks and fluxes, its excess N hypothesis is difficult to test. Therefore, NGA’s finding of an ecosystem N imbalance presents it with a fundamental challenge -- the interaction of multiple disturbances (e.g., multiple land use changes, fire, and N deposition) may have created a novel N environment for longleaf pine ecosystems, which hinders NGA’s ability to assess its capacity to respond to a historical fire regime.

NGA found surprisingly low rates of N fixation, and that the dominant contribution of N fixation differed by site (legume N fixation at Benning and asymbiotic N fixation at Eglin). This site difference appears to be driven by lower soil P availability and higher soil N availability at Eglin relative to Benning. Supporting this idea, legume N fixation increased with P addition in common garden and field fertilization experiments. Nitrogen fixation declined with N addition in the common garden experiment, but the degree of this response was species dependent. Although individual fire events temporarily stimulated N fixation by legumes, N fixation was insufficient to balance N losses from fire and soil N stocks declined over stand age.

Progressive N loss from the ecosystem may signal a decline in resiliency and present a long-term concern for land managers. An alternative possibility is that longleaf pine ecosystems have accumulated excess N as a result of land-use change and chronic N deposition. In this case, fire may be a relief mechanism for excess N, critical for returning the ecosystem to its N-poor state.

How longleaf pine (Pinus palustris Mill.) forests respond to fire and land use disturbance is an unanswered question, yet is critical knowledge for both land managers and ecologists. UGA states that this question remains unanswered because of its incomplete knowledge of how disturbance events influence transformations of nitrogen (N), which constrains carbon (C) accretion during forest regeneration (Nave et al. 2014). The recovery of forest ecosystems depends on the supply of mineral N, which is often dictated by the rate at which biological N fixation introduces new N (Rastetter et al. 2001).