Effect of Organic Mulches on the Content of Organic Carbon in the Soil

INTRODUCTION The content of organic carbon has long been recognized as a key component of soil quality (Reeves, 1997), and thus maintenance of soil organic carbon (SOC) in cropland soils is a major determinant of the productivity and long-term stability of agricultural systems (Carter, 2002). Soil organic matter (SOM) is composed of plant, animal, and microbial residues at varying stages of decomposition and amorphous humic substances (Jenkinson et al., 1992). An increase in the total content of SOC in organic farming was established by Marinari et al. (2010). Natural organic mulch eventually breaks down and becomes a part of the soil and a source of plant nutrients (Sharma et al., 1998; Bond & Grundy, 2001; Gruber et al., 2008). Increasing the amount of SOC is regarded as the main advantage of organic mulch (Paustian et al., 1997; Saroa & Lal, 2003). The influence of mulch on soil agrochemical properties depends on the chemical composition of mulch. It is recommended that in ecological farming soil should be covered with composts, chopped straw, and other organic residues to provide crops with nutrients, especially nitrogen (Relf, 2009). During the mineralization process of the mulch small amounts of nutrients become available for plants. However, it is not a sufficient supply of plant nutrients. Organic mulch is a source of nutrients for soil microorganisms, and as a result of their activity organic residues used for mulching are decomposed to available plant nutrients and a very important substance in soil--humus (Blanchart et al., 2006). Different opinions on the influence of organic mulches on SOM exist. A significant increase of SOC was observed in plots mulched with organic mulches (Relf, 2009). However, Kukkonen et al. (2004) stated that peat mulch has no significant influence on the amount of SOC. Oats and vetch grown for green manure and sprayed on soil surface were found to increase the amount of SOC and nitrogen in the soil compared with green manure inserted into the soil (Yadav, 1995; Astier et al., 2006). Cadavid et al. (1998) registered an increase of SOC and a decrease of the soil pH in plots covered with grass mulch. According to Blanco-Canqui & Lal (2007), mulching with straw during 10 years increased SOC by 33%. This means that 2/3 of the applied wheat straw was not converted into SOC and most probably was lost as emissions of C[O.sub.2] and C[H.sub.4]. Mulching even has the potential of reducing greenhouse gas emissions from soil by increasing its SOM content (Mulumba & Lal, 2008; Jordan et al., 2010). Our hypothesis was that the influence of organic mulch of different chemical composition on SOC will be unequal. The aim of the investigation was to evaluate the effect of various organic mulches and different thicknesses of the mulch layer on the SOC content. MATERIALS AND METHODS The two-factor field experiment was carried out in the Pomological Garden of Aleksandras Stulginskis University (formerly Lithuanian University of Agriculture) on a certified organic field (54[degrees]53'N, 23[degrees]50'E) in 2004-2011. The soil type of the experimental field is Calc(ar)i-Endohypogleyic Luvisol; its texture is medium clay loams on heavy clay loams and clays. Soil agrochemical characteristics are the following: [pH.sub.KCl] 6.4-6.7, the content of total nitrogen 0.119-0.142%, organic carbon 1.56-3.13%, available phosphorus 219.7-234.9 mg [kg.sup.-1], and potassium 134.3-180.5 mg [kg.sup.-1]. The sum of active temperatures (>10[degrees]C) in Lithuania is about 2100-2200[degrees]C, rainfall about 550-840 mm per year. The following treatments were used in the experiment: factor A--mulch: (1) without mulch, (2) straw, (3) peat, (4) sawdust, and (5) grass; factor B--thickness of the mulch layer: (1) 5 cm and (2) 10 cm. The direct influence of organic mulches was investigated in 2004-2009, and their residual effect in 2010-2011. …