SEASONAL CHANGES IN SOIL MICROBIAL BIOMASS AND ENZYME ACTIVITY IN ARABLE AND GRASSLAND SOILS


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SEASONAL CHANGES IN SOIL MICROBIAL BIOMASS AND ENZYME ACTIVITY IN ARABLE AND GRASSLAND SOILS

Hatice Sevim Uçkan ¹, Nur Okur ²

¹ Department of Soil Science, Faculty of Agriculture, Uludağ University, Görükle-BURSA
² Department of Soil Science, Faculty of Agriculture, Ege University, Bornova-İZMİR

ABSTRACT

Seasonal changes in soil moisture, soil temperature and C input from crop roots, rhizosphere products and crop residues can have a large effect on soil microbial biomass and its activity. The objective of this study was to quantify the seasonal changes in soil microbial biomass and the enzyme activity in soils under cultivation and grassland soils. Soil samples were taken at two different times (May and September, 1998). Five soils under cereal, five soils under vegetable and five grassland soils were selected for the study. All samples belonged to Alluvial Great Soil Group of Bursa Plain.

Significant differences were found between soil sampling periods for microbial biomass, mineralizable C and protease activity. Microbial biomass C in September compared to May averaged 52.5, 68.7 and 18.2 % greater under cereal, vegetable and pasture soils, respectively. Mineralizable C was higher in soils under cereal and vegetable in September, possibly due to more flows of C and N to soils. Dehydrogenase activity was similar in two different sampling periods. In May, more favourable weather conditions (moisture and temperature) and fertilization increased soil protease activity.

INTRODUCTION

Soil microbial biomass, a living part of soil organic matter, is an agent of transformation for added and native organic matter and acts as a labile reservoir for plant-available N, P, and S (Jenkinson and Ladd, 1981). The activity of the microbial biomass is commonly used to characterize the microbiological status of a soil (Nanniperi et al., 1990), and to determine the effects of cultivation (Beyer et al., 1991; Anderson and Domsch, 1993), field management (Perott et al.,1992), or contamination (Chander and Brookes, 1993) on soil microorganisms.

Soil physicochemical characteristics influence the level of biomass and the activity of microorganisms. Seasonal changes in soil moisture, soil temperature and C input from crop roots, rhizosphere products (i.e. root exudates, mucilage, sloughed cells, etc.), and crop residues can have a large effect on soil microbial biomass and its activity (Ross, 1987), which in turn, affect the ability of soil to supply nutrients to plants through soil organic matter turnover (Bonde and Roswall, 1987). Microbial biomass has been reported to vary seasonally in European soils (Patra et al., 1990). Singh et al. (1989) have also reported a seasonal variation in the microbial C, N and P in forests and savanna. Generally, a negative relationship between soil moisture content and microbial biomass was reported by Ross (1987) for New Zealand soils under tussock grassland and introduced pasture. Short-term fluctuations of moisture and temperature conditions have been shown to influence the amount of microbial biomass carbon (MacGill et al., 1986). Soil organic carbon levels, too, have been reported to be governed by climatic conditions (Jenny, 1980). Limited data, however, exist on the magnitude of seasonal changes in soil microbial biomass and enzyme activity in different agricultural ecosystems. In this study, the amounts of soil microbial biomass and enzyme activity in soils under cultivation were determined at May when crop root and residue additions were minimal and at September when crop residues were maximal. In addition, the same microbiological indices are also compared to those of grassland soils.

MATERIAL and METHODS

Sampling sites of soils : Fifteen Alluvial soil samples in the Northwest Anatolia, Bursa were sampled in May and September 1998. Annual temperature and rainfall averages 14.8 °C and 68.5 mm, respectively. Table 1 shows the crops and the characteristics of soils. Sand content of soils ranged from 17.12 to 53.84 %, clay from 22.00 to 42.16 % and silt from 20.00 to 50.00 %. Soil reaction (pH) was between 7.43 and 8.46, soil organic C content 0.52 and 1.71 % and the ratio soil organic C:total N 5.0 and 7.9.

Soil analysis : The collected soils were sieved (4 mm) and stored at 4 C° until needed. Subsamples for the determination of physicochemical parameters were air dried and sieved (2 mm) before analysis. Organic C was determined as described by Walkley and Black (1934) and, total N by Bremner (1960). Other physicochemical analysis were determined according to the standard methods.
Measurement of microbial biomass : Microbial biomass C was determined by subtrate-induced respiration method (Anderson and Domsch, 1978). Moist soil samples (100 g) were amended with glucose (400 mg), and the pattern of respiration response was recorded for 4 h. By a conversion factor, values were converted to mg biomass-C.
Mineralizable C : Mineralizable C was estimated from the quantity of CO2-C mineralized from soil during a 7-d incubation at 27 C° (Isermeyer, 1952).
Dehydrogenase activity : Dehydrogenase activity was determined according to the method of Thalmann (1967), in which TTC (2, 3, 5-triphenyl tetrazolium chloride) serves as a terminal acceptor of protons and electrons from organic compounds being oxidized.
Protease activity : Protease activity was assayed by the method of Ladd and Buttler (1972). This procedure involves the determination of aromatic amino acids released during the incubation period by using casein as substrate.

RESULTS AND DISCUSSION

Microbial Biomass and Mineralizable C : Microbial biomass C, mineralizable C and microbial biomass C: organic C (MBC:C) ratio are given Table 2. Soil microbial biomass C and mineralizable C increased significantly at 1 % level from the first sampling date (May) to the second sampling date (September) in all soils (Table 3). Microbial biomass C in September compared to May averaged 52.5, 68.7 and 18.2 % greater under cereal, vegetable and pasture soils, respectively. Since microbial biomass is generally correlated with the level of soil organic matter and is influenced by climatic variables, its solely measurement can not show whether a specific cropping or tillage is gaining or losing organic matter. In order to elucidate the changes in organic matter equilibrium, emphasis has been placed on the proportion of total organic C within the microbial biomass (Wu and Brookes, 1988; Anderson and Domsch, 1989). Using this concept, organic matter equilibrium functions have been provided for the ratio of biomass C to total organic C in a wide range of crop rotations (Anderson and Domsch, 1989) and macroclimates (Insam et al., 1989). In this study, the ratio of microbial biomass C to total organic C of soils are given Table 2.

MBC:OC ratio of every two sampling period differed significantly at 1 % level (Table 3). The ratio ranged from 5.3 to 9.9 in May while from 8.4 to 85.0 in September for all soil samples. These results showed that organic matter residues added to soils in September allows a more efficient organic matter utilization per unit biomass as compared to soils taken in May which is lesser in organic matter. When plant groups are compared, MBC:OC ratio of arable soils was higher than the pasture soils. The fact that two or more crops have been growing in arable soils caused a higher chemical diversity of organic matter input and the microorganisms which are endowed with a more economic metabolism are favoured with time (Anderson and Domsch, 1990).

Seasonal changes in the mineralizable C reflected the availability of easily-decomposable substrates. Mineralizable C was higher in soils under cereal and vegetable in September, possibly due to more flows of C and N to soils. C input from crop roots, rhizosphere products and crop residues can have a large effect on mineralizable C and N in soils. Robertson et al (1994) states a more flows of C and N in soils under sorghum than under green panic.

Enzyme Activities : A knowledge of the spectrum of enzymatic activities of a soil is important since it will indicate the potential of the soil to permit the basic biochemical processes necessary for maintaining soil fertility. Dehydrogenase and protease activities were determined in the study soils and the results obtained are given in Table 4.

No differences are found between the two sampling periods of dehydrogenase activity (Table 5). Dehydrogenases are considered to play an essential role in the initial stages of the oxidation of soil organic matter by transferring hydrogen and electrons from substrates to acceptors (Ross, 1971). Comparing plant groups, a higher dehydrogenase activity of pasture soils were found than the arable soils. There is a negative correlation between the dehydrogenase activity and the soil aerating conditions. In pasture soils where soil tillage is not applied, O2 input to soil is lesser compared to arable soils. Therefore, pasture soils contain lesser O2 than arable soils. The deficit aeration conditions in the pasture soils might explain the high dehydrogenase activity.

Significant differences were found between the two sampling periods of protease activity (Table 5). The values of protease involved in the N cycle were higher in soil samples which were taken in May 98. Soil proteases are rate-limiting enzymes in the N mineralization process of soils (Ladd and Paul, 1973) and negatively affected by aridity (Schinner et al., 1995). In May, more favourable weather conditions (moisture and temperature) and fertilization increased soil protease activity. Comparing plant groups, protease activity of pasture soils was higher than arable soils. The fact that soil is covered with alfalfa and vetch during all the year, probably, increased protease activity of pasture soils.

Relationships between microbial parameters and some soil properties : Although there was no relationship between soil organic C and microbial C, a significant correlation was found between soil organic C and the ratio of microbial biomass C to soil organic C (Table 6). Anderson and Domsch (1989) stated that the ratio of soil biomass c to organic C ratio is a good indicator of changes in microbial performance caused by environmental conditions. The structure and distribution of C in soil affect biological activity and probably the microbial biomass. Organic C and total N were highly correlated with dehydrogenase and protease activities of soils. Addition of N fertilizer or organic fertilizer to soils increased the microbial enzymatic activity.

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