Comparison of mineralisation and humification of postharvest residues of cereals in conventional and organic cropping practices

TESAŘOVa, M. , KUDLICKA, P., POSPISILOVa, L. , KALHOTKA, L. , HRABĚ, f. : Comparison of mineralisation and humification of postharvest residues of cereals in conventional and organic cropping. Acta univ. agric. et silvic. Mendel. Brun. , 2006, LIV, No. 1, pp. 121–126 Yearly inputs and transformations of above – and underground plant residues of winter wheat and spring barley were followed in „intensive“ (I, mineral fertilizers only) and „organic“ (0, most of nutrients applied as animal manure) crop sequences. Total amount of postharvest residues was lower in I crop system compared with that in the organic (O) one. Plant residues produced in O crop system were decomposed more rapidly both under field and laboratory conditions, than those from I crop system. Differences between Corg at the beginning and at the end of ten-weeks laboratory incubation of soils enriched by plant residues indicated that soil microflora humified more intensively a) spring barley than winter wheat residues and b) underground plant residues than straw. postharvest residues, winter wheat, spring barley, decomposition, humification, cropping practices One of the urgent problems of contemporary agriculture is the choice of prospective types of farming on arable soil in submontane areas. Such farming types should fulfil several conditions: they should be optimal with respect to yields, considerate with respect to natural environment, and they should observe traditional forms of farming of the respective region (Bouma, 2004; Schjonning et al., 2004). formulation of such agri-environmental policy therefore requires knowledge about the specific relationships that exist between farm management practices, base environmental conditions and field crops structure. These relationships are studied in the region of Bohemian Moravian Upland, where the economic and ecological aspects of using different crop sequences are tested. Among the evaluated criteria are both quantitative and qualitative changes of the organic soil matter as related to the balance of organic matter in the agroecosystem. Part of the study and its goal was to characterize the amounts of postharvest remains of cereals (winter wheat and spring barley) and the processes of their transformation in different crop sequences. MATERIAL AND METhODS field experiments were established in 1999 at the research station of the faculty of Agronomy, Mendel University of Agriculture and forestry Brno, at Vatin (Bohemian Moravian Upland, 530 m asl., average yearly air temperature 6.9 °C, average yearly precipitation 621 mm, soil type cambisol typical). There are two crop sequences each with six crops in rotation: 1) „intensive“ crop sequence (I) with exclusively cash crops (cereals, oil plants) and an optimal level 122 M. Tesařova, P. Kudlicka, L. Pospisilova, L. Kalhotka, F. Hrabě of chemical inputs (mineral fertilizers, pesticides) but without organic farmyard manure, 2) „organic“ crop sequence (O) with fodder and cash crops (clover, green-pea, cereals, potatoes), utilizing farmyard manure. Each experimental variant was established in four replicates, the size of individual plots was 12 m × 10 m. Both crop sequences involve winter wheat (var. Niagara) and spring barley (var. Nordus). Nutrients are applied in the „intensive“ crop sequence (I) exclusively in the mineral form at rations (kg/ha/year) 130N, 40P and 80K (winter wheat) and 60N, 35P and 80K (spring barley). Inputs of N, P, K in the „organic“ system (O) involve 90N, 30P and 80K to winter wheat and 40N, 30P and 60K to spring barley, however 60% of inputs are in the organic form. The mass of winter wheat and spring barley postharvest residues, the intensity of their decomposition and humification as well as selected parameters of the biological soil activity were recorded during the 2000 to 2002 years. The mass of postharvest residues was estimated immediately after the harvest in 2000 and 2001. Data on the mass of aboveground plant remains were obtained by both collecting from plots 20 cm × 20 cm (four replicates from each experimental variant), and by collecting from the surface of soil cylinders (see below). The mass of root remains was estimated in soil cylinders removed in 12 to 16 replicates for each experimental variant by means of a sampler 75 mm in diameter and 250 mm long. Aboveground plant remains were collected in the laboratory from the upper cylinder surfaces and then the root remains were isolated by gradual washing the cylindre on sieves with mesh sizes of 2 mm and 0.5 mm. Aboveground plant remains and root remains relieved of all adherent soil were then dried to constant weight at the temperature of 60 °C. The results were recalculated to the area of 1 ha. Microbial transformation of plant remains and the organic soil matter were monitored under both field and laboratory conditions. The rate of straw and root remains decomposition (litter-bag method), the CO2 output from soil and the intensity of cellulose decomposition were measured in the field by means of methods according to Tesařova (1987) and Tesařova et Gloser (1987). Soils sampled in 25 cm upper layer twice a year (spring and autumn of 2000, 2001 and 2002 years) were analyzed for Cox oxidimetrically by a Walkley-Black method (Nelson et Sommers, 1982) and numbers of cellulolytic microorganisms (plate counts method). Laboratory experiments were focussed on the processes of mineralization and humification of plant remains. freshly collected soil samples were sieved (2mm mesh) and enriched by plant remains (100 g soil, 2 g of finely cutted straw or roots) and incubated for 10 weeks under optimal temperature (25 °C ± 1 °C) and moisture conditions (60% WHC). The output of CO2. was recorded during the whole time of incubation using soda-lime as absorbent (see Tesařova et Gloser, 1987). Soil samples taken at the beginning and on the end of incubation were dried in the air, sieved at mesh size of 0.5mm and analyzed for the Corg. The data were statistically evaluated (Statgraphic, ANOVA P=0.05). RESULTS AND DISCUSSION The overall amount of postharvest remains passing every year into the soil i.e sum of stubble, straw and root residues, has reached 5.8 to 7.5t per ha– 1 for winter wheat and 2.9 to 3.6t per ha– 1 for spring barley (Tab. I). Similar values were published by Jurcova et Bielek (1997): 5.6 and 3.97 t.ha– 1 for winter wheat and spring barley respectively. The postharvest residua of both crops involved about 20 to 30% of the roots (Tab. I). This is evidently the reason why no relationship was found between the total amount of postharvest remains and the yields. The yields of above – ground plant biomass, i.e. sum of grain and straw attained with winter wheat in the I (intensive) and O (organic) crop sequences 10.05 t.ha– 1 and 9.29 t.ha– 1 respectively, corresponding data for spring barley were 7.01 t.ha– 1, and 6.49 t.ha– 1. Root remains of cereals in soil were decomposed under field conditions substantially more slowly than the straw (Tab. II). Whitmore et Matus (1996) clearly showed that the representation of fibre carbon (hemicellulose plus cellulose plus lignin) is more higher in root than in shoot residues of cereals. As easily decomposable parts of residues break down, the fibrous part become more concentrated in the remainder and retard decomposition to a greater and greater extent. The decomposition rate of both root remains and straw was always higher (statistically significant in most cases) in the O than in the I crop sequence. Same differences in the decomposition rate of root remains (evaluated according to the CO2 output) were found between cereals grown in both O and I crop rotations under laboratory conditions (fig. 1). One reason of the more intensive decomposition rate of postharvest cereal remains from the O crop rotation can be the more favourable C/N ratio. The values of C/N ratio for wheat straw from the O crop rotation attained 66 and from the I crop rotation 72. Similar relations were found for the barley straw (C/N = 55 for I and 50 for O crop sequence). Comparison of mineralisation and humification of postharvest residues of cereals 123 I: Average mass of postharvest cereal remains (t.ha–1 per year)