Interaction between gas exchange rates, physical and microbiological properties in soils recently subjected to agriculture
Introduction
The understanding of the subtle interplay between microbial metabolism and the physical aspects of soil organization is crucial for a thorough comprehension of soil function. Soil microbial biomass comprises 1–4% of the total organic carbon (Anderson and Domsch, 1985), and 2–6% of the total organic nitrogen (Jenkinson, 1988) in soil. Microorganisms, despite their relatively low amounts, play the crucial role of keeping the main nutrient cycles in soil (C, N, P and S) through recycling from organic matter (OM). The latter is fundamental not only for primary production but for the long-term functioning of ecosystems as well (Stevenson, 1986, Doran and Parkin, 1994, Doran and Parkin, 1996).
Respiration is a process that reflects biological and biochemical activity being put in evidence through the production of CO2 or consumption of O2 as a result of the metabolic processes in living organisms. As an indicator of OM decomposition in soil, soil respiration reflects two general processes: (i) loss of C from the soil system; (ii) recycling of nutrients (Parkin et al., 1996).
Soil respiration reveals the biological activity of the entire soil biota including microorganisms, macroorganisms (such as earthworms, nematodes, and insects), and plant roots. On the other hand, microbial respiration is characterized by the production of CO2, or the uptake of O2 as a result of microbial metabolism (bacteria, fungi, algae, and protozoa) (Parkin et al., 1996, Aon and Cortassa, 1997). The latter is a function not only of the density of microorganisms but also of their metabolic status which in turn depends on soil physico-chemical conditions such as temperature, porosity or water content (Doran, 1980, Alexander, 1991, Russell, 1992, Paul and Clark, 1996).
The soil air/water balance determines the relative extent of aerobic and anaerobic microbial activity. The soil aeration critically depends upon physical conditions such as bulk density and pore size distribution. Existing experimental evidence indicates that soil water content is an important factor that influences microbial type and extent of metabolism (Linn and Doran, 1984a, Linn and Doran, 1984b). The water content of soil exerts its influence through the regulation of oxygen availability and thus microbial carbon (oxidative or fermentative) and nitrogen (ammonification, denitrification) metabolism. Aerobic microbial activity increases with soil water content until a point is reached where water displaces air and restricts the diffusion and availability of oxygen (Miller and Johnson, 1964, Linn and Doran, 1984b). The water-filled pore space (WFPS) denotes the well-known relative soil water content, i.e. the ratio of volumetric water content over the total soil porosity. At high WFPS, soil pores are mainly occupied by water which favors anaerobiosis and consequently anaerobic microbial metabolism. Metabolically speaking, when cells utilize oxygen as electron acceptor, an oxidative type of substrate breakdown happens and a more exhaustive degradation of OM may be expected. Under more anaerobic conditions, other electron acceptors such as NO3−, Mn(IV), or Fe(III), may be used (Paul and Clark, 1996). Under these conditions, OM decomposition still occurs and an active secretion of metabolites by microorganisms can happen, e.g. organic acids, together with an increase of the RQ through high CO2 production by facultatively anaerobic microorganisms (Aon and Cortassa, 1997).
An important aspect of quantifying gas exchange rates (specially CO2) in soil, concern the possibility of evaluating the contribution of agricultural practices to the greenhouse warming. It is known that the short-term changes in CO2 flux from soils after plowing are very important (Reicosky and Lindstrom, 1993). The CO2 flux decreased from a maximum of 114–48 g CO2 m−2 h−1 within a lapse of 8 min and up to 8 g CO2 m−2 h−1 at 3.5 h. All these fluxes were much higher than the average CO2 fluxes (0.38 g CO2 m−2 h−1) from no-tillage plots (Reicosky and Lindstrom, 1993).
Determination of soil respiration has been mainly performed in the laboratory, indicating that maximum aerobic microbial respiration occurs when 60% of the soil pores are filled with water (Linn and Doran, 1984b, Doran and Linn, 1994, Parkin et al., 1996, Paul and Clark, 1996). However, the relationship between microbial respiration and WFPS has not been extensively evaluated in the field (Parkin et al., 1996). In the present work, we apply a methodology developed by ourselves (Cortassa et al., 2001) to measure the capacity of soils to either consume O2 or produce CO2 at a certain rate, as a function of soil water content, under conditions as close as possible to those of the field. Correlation of soil respiration measurements with microbial abundance, physiology, and distribution as a function of depth and the air/water balance, are also attempted.
Section snippets
Materials and methods
The soil characteristics as well as management practice and experimental design are described in Aon et al. (2001a). Briefly, the soil examined (Hapludoll) belongs to the El Salado river basin (Buenos Aires, Argentina) with a high content of OM (3–5%) and relatively acid pH (between 5.5 and 6.0), corresponding to natural grasslands recently incorporated to agriculture.
Soil samples were extracted from a controlled experience of plots subjected to no-till (NT) or conventional tillage (CT) or
Results and discussion
We have investigated a soil from the El Salado river basin (Buenos Aires, Argentina) recently subjected to agriculture. The latter represents an ideal situation since it allows us to investigate more thoroughly initial stages of soil quality deterioration induced by different agricultural practices (e.g., tillage versus no-tillage, fertilizers) (see Aon et al., 2001a).
As a step toward that goal, we applied a method developed in our laboratory that measures the atmosphere composition in O2 and CO
Acknowledgements
We thank Prof. Dr. J.W. Doran (University of Nebraska, USA) for useful criticism on an earlier version of this manuscript and Agr. Eng. Rodolfo C. Gil (INTA Castelar, Argentina) for helpful discussions and suggestions concerning physical measurements in soil. The authors are specially indebted with Fundación Antorchas, Argentina, for a grant with which the Gas Analyzer, Columbus, OH, used in the present work was acquired. MAA and SC are research scientists from CONICET, Argentina; DES is
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