Soil quality variables in organically and conventionally cultivated field sites

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Abstract

This study aimed to answer the following questions (a) which of the soil variables (chemical, biological, enzyme activities and potential metabolic profile based on the Biolog method) could be used as indicators reflecting differences in soil quality between organically and conventionally managed asparagus fields, (b) how the duration of organic management affects these soil variables and (c) in what extent the soil quality in organic fields is comparable to that in hedgerows. The study included four organically cultivated fields which differed in the time they enter organic treatment: 6 years (O6), 5 years (O5), 3 years (O3) and 2 years (O2), the closest to them hedgerow (Ho), a conventionally managed field (CF) and its adjacent hedgerow (Hc). Among the chemical and biological variables, those contributing for most to the discrimination of the organic and conventional fields were mainly microbial biomass C (MBC) and N (MBN) and secondly variables related to N-cycle (NO3, N organic, rate of N mineralization). MBC and MBN were higher in organic and conventional areas, respectively, reflecting differences in the structure of their microbial communities. The chemical and biological variables did not differ among O3, O5 and O6 fields, while low values of organic N, rate of N mineralization and extractable P was recorded in O2 area. The enzyme activities of amidohydrolases (l-asparaginase,l-glutaminase, urease) and phosphatases (alkaline and acid phosphatase) were by far higher in organic areas than in the conventional one. The activities of amidohydrolases and alkaline phosphatase changed in a similar way, exhibiting higher values in O3 and O5 areas while between the oldest (O6) and the newest (O2) area no differences were recorded. While the suppress of enzyme activities in O2 was related to low inputs due to transition, the negative feedback between supply of N and P and activities in O6 was a possible explanation. The activity of acid phosphatase increased from the newest to oldest organic areas. The potential functional diversity and substrate evenness did not differ among sampling areas. Differences between areas were recorded only in relation to the carbohydrates' consumption. The soil quality of hedgerows seems to be completely different than that of the arable land in terms of all studied variables. Also, significant differences were recorded between the two hedgerows, a fact that could be related to the different management practices applied in the neighboring fields.

Introduction

Alternative agricultural practices such as organic farming have been initiated mainly since 1990. There is growing evidence that organic systems exhibit an improved soil quality characterized by higher biological activity than conventional ones (Drinkwater et al., 1995, Droogers and Bouma, 1996). In organic systems, plant production depends almost exclusively on nutrient transformations in soils, since only limited amounts of permitted fertilizers are used. Because nutrient transformations are primarily controlled by microbes, an active soil microflora and a considerable pool of accessible nutrients are very important to the smooth functioning of organic systems.

The evaluation of soil quality is quite complex and requires the consideration of physical, chemical and biological variables (Wander and Bollero, 1999), since the different variables exhibit disproportional behavior when the land use changes (Bending et al., 2004). Specifically, enzyme activities have been suggested as good early indicators of changes in soil properties because of their relationship to soil microflora, the easiness of measurement and their rapid response to changes in soil management (Dick, 1994). Moreover, the substrate utilization patterns of the soil microbial communities are known to be sensitive to management practices (Marx et al., 2001) while their functional diversity could provide information on the resilience of these communities to stress.

Most frequently comparisons of soil quality are made between tillage and no tillage systems (Aon and Colaneri, 2001, Carpenter-Boggs et al., 2003). A factor usually neglected in this studies is how the duration of organic management (e.g. the time since the organic cultivation begun) affects soil properties (Werner, 1997), although it is known that soil functioning changes only after some years of organic farming (Drinkwater et al., 1995). Soil function in organic systems is based upon organic matter decomposition. From this point of view they can be comparable to hedgerow systems, which constitute islands of natural vegetation between cultivated areas. Although it is expected that the nearby to hedgerows land use exert a strong impact on their dynamics, only in a few studies was the soil quality of cultivated areas (organic and conventional) compared with that of their neighboring hedgerows (Aude et al., 2003).

With the above considerations, this study aims at answering questions relating to the application of organic management. Specifically, the addressed issues were (a) which of the tested soil variables could be used as indicators reflecting differences in soil quality between organically and conventionally managed asparagus fields, (b) how the duration of organic management affects soil variables, (c) is soil quality in organic fields comparable to that in hedgerow systems (d) does the type of management affect the soil variables of the nearby hedgerows.

Section snippets

Study area

The study area is located in Kria Vrisi of the Pella prefecture, 75 km west of Thessaloniki, Greece. The parent rock consists of Alluvial deposits (Holocene) (IGME, 1983). The mean pH (H2O) value of the study area was about 8, while the increasing order of soil electrical conductivity was organic areas<conventional area< hedgerows (Table 1). All cultivated areas were planted by Asparagus officinalis (L.) plants. The organic areas were nearby to each other consisting a continuous field area

Chemical and biological variables

In Table 2 the average values of soil chemical variables are given. ANOVA showed that the concentrations of organic C, organic N and extractable P differed significantly among the sampling areas, while differences in the inorganic N (NH4 and NO3) concentrations were not significant. From comparisons between the organic fields, we found that O2 exhibited the lowest amounts of organic C, organic N and extractable P, while no differences were recorded among the rest of the organic areas. The

Chemical and biological variables

Based on soil chemical and biological variables, sampling areas were discriminated clearly consisting three different groups: the organically managed areas, the conventionally cultivated area and the hedgerows. Of the six variables that contributed significantly to the discrimination, MBC and MBN were of highly importance. The rest significant variables related to N transformation and to fungal biomass. Higher amounts of MBC and MBN characterized the organic systems (OF) and the conventional

Acknowledgements

This research was supported by the Greek General Secretariat for Research and Technology and the European Union under the program PENED-2001 with Grant# 01EΔ431. We are special grateful to R.G. Joergensen who offered expert assistance on ergosterol determination and J.M. Halley for his contribution to the linguistic corrections of the text.

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