Effects of moisture and temperature on net soil nitrogen mineralization: A laboratory study

Abstract

Climate change will lead to changes in soil moisture and temperature, thereby affecting organic matter mineralization and the cycling of biophilic elements such as nitrogen. However, very few studies have considered how the sensitivity of the rate of net nitrogen mineralization to temperature and/or moisture content may be modified by changes in these parameters. To investigate how changes in temperature and moisture content affect net nitrogen mineralization (as regards both the mineralization rate and the sensitivity of the mineralization rate to changes in temperature and moisture content), a laboratory experiment was carried out in which three soils under different types of use (Forest, Grassland, Cropland) were incubated for 42 days under different moisture conditions (between 40 and 100% field capacity) and temperatures (between 10 and 35 °C); total inorganic nitrogen levels were determined at different times throughout the experiment. The rate of mineralization was determined at each temperature and moisture level considered, by use of the mono-compartmental model developed by Stanford and Smith (1972). For all soils, changes in the rate of mineralization with temperature followed the pattern described by the Q₁₀ model, while the models used to determine the effect of moisture content on the net rate of mineralization (linear, semilogarithmic, partial parabolic and complete parabolic) were only verified for the Forest soil. In general, the sensitivity to temperature was maximal at 25 °C, and the optimal moisture content for nitrogen mineralization was between 80% and 100% of field capacity. A relatively simple model that included the temperature–moisture–time interaction was also tested. This model provided a significant fit for the three soils under study, in contrast with the other models tested. In any case, further studies are necessary in order to address the extent to which changes in the quality of organic matter, caused by land use, affect any modifications to soil nitrogen that may be generated by climate change.

Introduction

Climate is changing on a global scale and affecting soil temperature and moisture regimes [1]. Climate change will therefore also affect all edaphic processes that depend on soil temperature and moisture, including soil organic matter mineralization [2], [3]. There has been great interest in the possible effects of climate change on soil carbon cycling and on CO₂ emissions [4], [5]. There are two main reasons for this interest. Firstly, as CO₂ is one of the main greenhouse gases (i.e. it is capable of affecting climate change), it is essential to understand the feed-back mechanisms between climate change and CO₂ emissions. Secondly, measurement of CO₂ emissions is a straightforward, rapid method of determining the impact of climate change on edaphic metabolism [4].

However, carbon is not the only element affected by climate change, and elements such as nitrogen, phosphorus and sulphur, typically associated with organisms, are also affected. All of these are essential elements and are therefore extremely important for the correct functioning of ecosystems. In the case of nitrogen, most bioavailable forms of this element in natural soils are produced by mineralization of organic matter via depolymerization of large organic polymers, so that the productivity of many ecosystems will depend directly on the availability of nitrogen derived from the decomposition of organic remains. It is therefore reasonable to assume that any modification that affects the rate of decomposition of organic matter will also affect the availability of nitrogen, and therefore will have important repercussions for ecosystem functioning [6].

Many studies involving mineralization of organic nitrogen compounds in soil have been undertaken since the early studies in 1972 by Stanford and Smith, as reported by Dessurealt-Rompré et al. [7]. Stanford and Smith [8] demonstrated that net nitrogen mineralization followed first order kinetics and that the effect of temperature on nitrogen mineralization followed the Arrhenius law, with the rate doubling for each 10 °C increase in temperature. Various authors have recently discussed the effects of temperature and/or moisture content on the net mineralization process, as well as how soil moisture should be expressed (percentage of pore filled, water potential, etc.) and how the different mathematical models used to study such relationships should be interpreted [2, 7, 9, 10, 11, amongst others]. Most of these studies have focused on the rate of nitrogen mineralization, with the aim of estimating the amount of nitrogen that is available to plants. In other words these studies had an agronomic focus. Nevertheless, very few studies about soil nitrogen mineralization consider the sensitivity of the mineralization rate to changes in temperature and moisture with the aim of determining the direct impact of climatic parameters on the nitrogen cycle [7]. This is an important omission as more accurate information about the sensitivity of mineralization constants and how they vary in relation to climatic parameters would enable better description of the response of soil organic matter to climate change. Furthermore, there is even less information about the combined effects of these two factors (temperature and soil moisture) on net nitrogen mineralization processes [11], [12], [13], despite evidence that climate change will affect both components of the climate simultaneously, as indicated above. In fact, Craine and Gelderman [14] indicated their surprise about the scarcity of information as to how soil moisture affects the temperature sensitivity of soil organic matter decomposition and whether or not any such relationships are consistent across different soils.

At present the climate in Galicia (NW Spain) is humid temperate and, because of its geographical position (north of 40° N), the region is expected to undergo significant changes [1]. It is therefore important to predict the response of soils to such changes, and to establish which types of soils (in terms of management regimes) will be most affected. Recent laboratory and field studies in Galicia have addressed the issue of how CO₂ emissions in soils will be affected by climate change [15]. The thermodynamic properties of several edaphic enzymes (both hydrolases and oxidoreductases) have also been characterized in order to establish how decomposition processes mediated by these enzymes would be affected by changes in soil temperature [16]. However, to date the effects of climate change on nitrogen mineralization have not been analyzed in Galician soils. Therefore, and given the general lack of knowledge about the influence of climate changes on net nitrogen mineralization, the objectives of the present study were: a) to investigate how net nitrogen mineralization is modified in response to changes in temperature or moisture, b) to determine the extent to which land use modifies the sensitivity of net nitrogen mineralization to changes in soil temperature and moisture, and c) to investigate the extent to which climate change will affect nitrogen availability in Galician soils.