Effects of chronic nitrogen amendment on dissolved organic matter and inorganic nitrogen in soil solution

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Abstract

Increased atmospheric deposition of N to forests is an issue of global concern, with largely undocumented long-term effects on soil solution chemistry. In contrast to bulk soil properties, which are typically slow to respond to a chronic stress, soil solution chemistry may provide an early indication of the long-term changes in soils associated with a chronic stress. At the Harvard Forest, soil solution was collected beneath the forest floor in zero tension lysimeters for 10 years (1993–2002) as part of an N saturation experiment. The experiment was begun in 1988 with 5 or 15 g N m−2 per year added to hardwood and pine forest plots, and our samples thus characterize the long-term response to N fertilization. Samples were routinely analyzed for inorganic nitrogen, dissolved organic nitrogen (DON), and dissolved organic carbon (DOC); selected samples were also analyzed to determine qualitative changes in the composition of dissolved organic matter. Fluxes of DOC, DON, and inorganic N were calculated based on modeled water loss from the forest floor and observed concentrations in lysimeter samples. The concentration and flux of inorganic N lost from the forest floor in percolating soil solution are strongly affected by N fertilization and have not shown any consistent trends over time. On average, inorganic N fluxes have reached or exceeded the level of fertilizer application in most plots. Concentrations of DOC were unchanged by N fertilization in both the hardwood and pine stands, with long-term seasonal averages ranging from 31–57 mg l−1 (hardwood) and 36–93 mg l−1 (pine). Annual fluxes of DOC ranged from 30–50 g m−2 per year. DON concentrations more than doubled, resulting in a shift toward N-rich organic matter in soil solution percolating from the plots, and DON fluxes of 1–3 g m−2 per year. The DOC:DON ratio of soil solution under high N application (10–20) was about half that of controls. The organic chemistry of soil solution undergoes large qualitative changes in response to N addition. With N saturation, there is proportionally more hydrophilic material in the total DON pool, and a lower C:N ratio in the hydrophobic fraction of the total DOM pool. Overall, our data show that fundamental changes in the chemistry of forest floor solution have occurred in response to N fertilization prior to initiation of our sampling. During the decade of this study (years 5–14 of N application) both inorganic N and dissolved organic matter concentrations have changed little despite the significant biotic changes that have accompanied N saturation.

Introduction

Soil solution chemistry is often a sensitive indicator of biogeochemical processes in forests, responding quickly to various disturbances or stresses. Early experiments by Vitousek et al. (1979), for example, showed that forest cutting results in a rapid change in soil solution chemistry, particularly inorganic nitrogen. Much of the literature on the effects of disturbance on forest soil solution chemistry and fluxes over the past two decades has focused on inorganic nutrients (e.g. Edwards and Ross-Todd, 1979, Blood et al., 1991, Knight et al., 1991, Prescott, 1997). Recent work shows that dissolved organic matter (DOM) plays a significant role in many aspects of forest biogeochemistry such as weathering (Adamo and Violante, 2000), soil genesis (Lundstrom et al., 2000), and plant nutrition (Lipson and Nasholm, 2001). With this recognition of the importance of DOM in forest biogeochemistry, interest in understanding the dynamics of dissolved organic matter in the face of chronic or episodic disturbances has also increased, as has interest in understanding the linkages between dissolved organic matter and inorganic nutrient dynamics, particularly for nitrogen (Vitousek et al., 1998). Studies of the effects of increased nitrogen deposition on forest biogeochemistry provide excellent opportunities to examine these linkages. In many soils, the dominant form of N in transport is organic N prior to experimental manipulation of N inputs, but shifts to inorganic forms with increased anthropogenic N inputs, providing an opportunity to examine the role of N availability in DOM dynamics (Aber et al., 1998).

Dissolved organic matter in soil solution is generated by a variety of pathways, and understanding the primary drivers of DOM production is a major challenge for forest ecologists and biogeochemists (Kalbitz et al., 2000, McDowell, 2003). Both biotic and abiotic processes are important in regulating dissolved organic carbon (DOC) and nitrogen (DON) flux (Currie and Aber, 1997, Aitkenhead-Peterson et al., 2003). Detailed analysis of the components of DOM (amino acids, carbohydrates, amino sugars) indicates that most is of microbial origin, derived from the oxidative degradation of plant-derived organic matter and by production of microbial metabolites (Guggenberger et al., 1994, Schimel and Weintraub, 2003). Work by Møller et al. (1999) suggests that fungal community composition is an important controller of DOC production, and large increases in DON have been attributed to cell death and lysis after soil is treated with biocide (Yavitt and Fahey, 1984).

Most disturbances or manipulations tend to increase the concentration of DOC and DON in forest soil solution (Kalbitz et al., 2000), with effects that are typically mediated by microbial processes and last a year or 2. Forest cutting, organic matter additions, liming, acidification, and hurricanes have each been shown to increase DOM concentration and flux in forest floor solution (Sollins and McCorison, 1981, Guggenberger, 1994, Johnson et al., 1995, McDowell et al., 1996, Qualls et al., 2000, Kalbitz et al., in press). In contrast to the consistent increases in DOM seen with other disturbances, studies of the effects of N fertilization on DOM dynamics have yielded inconsistent results. In Europe, the NITREX sites showed little change in DOC and DON production and export in soil solution with addition of N for 4–6 years (Emmett et al., 1998, Gundersen et al., 1998, Stuanes and Kjønass, 1998, Raastad and Mulder, 1999). Similarly, laboratory incubations of soils collected from sites with 8–29 years of N fertilization showed no effects of N fertilization on DOC or DON production (Sjöberg et al., 2003). In contrast, a chronic N fertilization experiment in southern Norway showed that after 9 years of NH4NO3 additions (at 3 and 9 g m−2 per year) there was a significant difference between control and treatment plots in both DOC and DON concentrations (Vestgarden et al., 2001). Forest floor DOC concentrations declined from 80.3 mg l−1 in control plots to 24.9 and 38.5 mg l−1 in the low and high N plots, respectively; similar results were obtained for DON. In Massachusetts, after 7 years of chronic nitrogen addition, McDowell et al. (1998) and Yano et al. (2000) reported no significant change in DOC concentrations, or the fraction of DOC that was biodegradable. They did, however, observe large increases in DON concentrations (double or triple the control values). The reasons for these widely divergent results (no change in DOC or DON at one set of European sites; declines in both DOC and DON at another; and no change in DOC with large increases in DON at a North American site) are unknown, suggesting that there is much yet to be learned about the processes driving production and uptake of DOC and DON in forest soils.

Most of the literature on the response of DOM to forest disturbance is based on sampling soil solution for a few years. The extent to which disturbance responses are maintained over intervals longer than a few years is largely unknown. Here we present data from ten years of sampling soil solution at the Harvard Forest chronic N experiment. We hypothesized that DOC and DON concentrations and flux would show progressive changes in response to the declines in ecosystem health predicted to occur with chronic N fertilization.

Section snippets

Study site

The Harvard Forest, Petersham, Massachusetts, USA (42°30′N, 72°10′W) is the site of the chronic N experiment. The region has undergone extensive changes in vegetation and land use over the last two centuries, with a cycle of forest clearing, agricultural or woodlot use, abandonment, and forest regrowth (Foster, 1992). Average monthly temperatures range from −7 °C in January and 19 °C in July; precipitation is evenly distributed throughout the year and averages 1100 mm (Currie et al., 1996).

Dissolved inorganic nitrogen

Concentrations of NH4+ and NO3 did not show any consistent trends over the 10-year study period in either the hardwood or pine plots (Fig. 1). Ammonium and nitrate both increased substantially in the hardwood high N plot from 1993 to 1996, for example, but dropped almost as much from 1996 to 2001. Soil solution from the low N treatments was consistently around 4 mg l−l NH4-N, and 6 mg l−l NO3-N, and also showed no strong trends over time in either the hardwood or pine plots. Flux from the forest

N saturation

Our results show that the organic soil horizons in both the pine and hardwood plots have been effectively “saturated” (Aber et al., 1989) with N for at least the past decade, when our sampling began. For the pine plots, average output of inorganic N in forest floor solution exceeded N inputs by fertilization for both application rates (Fig. 5). For the hardwood plots, outputs were similar to or exceeded inputs when the dissolved organic N resulting from fertilization was included in the N

Acknowledgements

This is Scientific Contribution Number 2219 from the New Hampshire Agricultural Experiment Station. In addition to the Agricultural Experiment Station, support was provided by the National Science Foundation Long-term Ecological Research Program to Harvard Forests and the USDA National Research Initiative Competitive Grants Program. We thank K. Kalbitz for a particularly thorough review of the manuscript. Y. Yano, W. Currie, and a legion of dedicated undergraduate researchers provided

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