Microbiological indicators for quality of soils at various stages of degradation in the forest-savanna-transition zone, south-western Nigeria

Abstract

Soil quality or soil health has emerged as the central concept for examining and integrati1lg relationships and functions among various biological, chemical and physical parameters of soils which are important in the context of sustainable land use and management. Identifying appropriate quantitative criteria and methods for assessing soil qual ity is a primary requirement to advance the concept. A minimum data set was already propost:d and basic soil chemical and physical properties determined. However, only a few biological aspects of soil quality were included so far, making the identification of biological indicators of soil quality as critically important. The present study was undertaken ta link sail micrabialagical and sail biachemic parameters with so ii quaiity conditions and crop productivity and to identi f y those parameters or processes that were affected most by iong-term management. Degradation was defined by iand-use history and was reflected in the so ii quaiity status and the productive potential. A degradation index was used to discriminate between three selected sites varying in time and intensity of land use based on the continuously cropped controls 0[ long-terrn experiments. Various improved [allow management systems were evalualed [or their potential as low-input continuous crop production systems by comparing them 10 sole cropping. The experimental sites were located at the research farm of the International Institt!te of Tropical Agriculture (IIT A) in south-western Nigeria. Three sites varying in degree of degradation and land use history were examined. The non-degraded Westbank 3 site ( 1 ) was established in 1989 and cleared from secondary forest. Three fallow management systems with maize/cassava intercropping -natural regrowth of the spontaneous vegetation, alley cropping with leucaena [Leucaena leucocephala (Lam.) de Wit], and pueraria in situ ("live") mulch (Pueraria phaseoloides) were introduced. The degraded D 2 site (2) was used by breeders between 1980 and 1985, and since 1986 cropped to two alley cropping managements with maize/cowpea sequentia1 cropping -leucaena and sellna [Senna siamea (Lam.) Irwin&Bameby]. The most severely degraded Westbank) site (3) was estab1ished in 1979 and was used under intensive mechanized cropping for a period of 10 years. Between 1989 and 1993 woody and herbace! ous fallow species were plallted to biologically restoring a severely degraded Alfisol. In 1993 one third of the plots were cleared and cropped to maize/cassava intercropping. The fallow species investigated comprised pueraria, leucaena, senna and natural regrowth of the spontaneous vegetation Continuous cropping at all sites was used as control. Between April 1993 and October 1994 composite soil sampIes were taken at random evcry 6 weeks from the 0-5 cm and 5-10 cm depth. In the leucaena and senna alley cropping treatments sampling was done along the hedgerows and in the interrow space. They were analyzed for bulk density, particle size distribution, gravimetric soil moisture content, pH, exchangeable basic cations, inorganic and organic phosphorus pools, total organic carbon and total nitrogen, microbial biomass carbon, acid and alkaline phosphatase, ß-glucosidase, and protease activity. The characterization of the study sites and their degree of degradation was assessed by analyzing continuous cropping controls. It showed for ( 1) WB 3: medium maize productivity, high levels of soil organic matter related properties, low phosphorus status and acid phosphatase activity, high microbial biomass content, alkaline phosphatase and protease activity; (2) D 2 was characterized by high maize productivity, low levels of soil organic matter related properties, high phosphorus status, low microbial biomass content as well as low alkaline phosphatase and protease activity. Characteristic for (3) Westbank 1 was low maize productivity, low levels of soil organic matter related properties, low inorganic phosphorus status and high organic phosphorus content at 0-5 cm depth, high acid phosphatase activity, low microbial biomass content and alkaline phosphatase activity as well as low protease activity. Differences between the degraded D 2 and Westbank 1 sites as reflected in maize productivity were largely due to inorganic Bray-I phosphorus at both depths and organic phosphorus at 5-10 cm depth. Differences in relation to soil microbiological properties were largely due to alkaline phosphatase activity. The effects of improved fallow management systems on site degradation were as follows: (1) at Westbank 3 leucaena was considered less effective in improving soil conditions than pueraria, as only total N of the soil chemical properties and both ß-glucosidase and acid phosphatase ( 1994 in the top layer) activities of the microbiological properties were maintained at a higher level as compared to continuous cropping. Pueraria maintained consistently and significantly highest values of the soil chemical and microbiological properties, except for maize productivity, pH, exchangeable basic cations, and inorganic phosphorus; (2) at D 2 leucaena alley cropping generally was not different in crop productivity, soil chemical as well as soil microbiological properties over sole cropping. Senna performed slightly better compared to leucaena. Organic carbon and microbial biomass content, alkaline phosphatase and protease during the rl season, and ß- glucosidase activity during the lst (both depths) and 2nd (0-5 cm) season were improved, crop productivity was not different from sole cropping; (3) at the most degraded Westbank 1 site, pueraria and natural regrowth! performed poorest and only improved total C and N (and pH under natural regrowth) when compared to continuous cropping. Leucaena fallowing for 4 years significantly enhanced maize yield at p = 10 %, total C, N, pH, and exchangeable basic cations. Microbial biomass content, alkaline phosphatase, and ß- glucosidase activity were significantly increased in the top 5 cm, whereas protease acti vity was only increased during the lst season at 0-5 cm depth. Most successful for improving soil productivity and fertility was the introduction of the senna faJlow. Maize grain, pH, total C and N, exchangeable basic cations, Bray-I phosphorus (rt season), NaOH- extractable organic phosphorus as well as microbial biomass content (0-5 cm), alkaline phosphatase, ß-glucosidase, and protease activity ( lst season and dry season at 5-10 cm depth) were improved significantly over the continuous cropping controls. Indicators appropriate to assessing soil quality should ( 1) reflect major processes or controlling factors which would affect sustainability at the sites, (2) be measurable against some definable standard, (3) be sensitive enough to detect differences in time and space, and ( 4) reflect cause-effect relations. Of the soil microbiological parameters analyzed, microbial biomass content, alkaline phosphatase, and ß-glucosidase activity were the most sensitive and consistent indicators to meet these requirements. By means of P CA it could be demonstrated that SOM-related nutrient dynamics was the major contributor to explaining the total variance (> 80 %) of the resource base under the prevailing experimental conditions. Microbial biomass, ß-glucosidase and alkaline phosphatase activities are parameters that are strongly associated with SOM and its turnover in soil. Highest loadings with the major PC were provided by microbial biomass and alkaline phosphatase, but also ß-glucosidase had loadings above 0.8. The ld PC could be interpreted as "phosphorus dynamics". By plotting the factor scores against both p Cs, the differentiation of the sites and treatment effects could be improved. Contrasting soil and crop management systems (alley cropping, live mulch, planted fallow, controls in long-term experiments) at three sites differing in degree of soil degradation could be assessed adequately by these indicators. Of the soil enzymes measured, ß- glucosidase activity was a sensitive indicator of the effect of improved fallow managements (alley cropping, live mulch, planted fallow) on site degradation. The bioassay of this enzyme was more sensitive in indicating changes as compared to total organic carbon. Alkaline phosphatase was more sensitive than microbial biomass in characterizing the sites and their degree of degradation as weIl as the effects of improved fallow managements on site degradation. The temporal fluctuations in the activity were much less pronounced tban for microbial biomass, wbich fluctuated bigbly. Since botb enzymes bad strong correlations with microbial biomass, they indicate the metabolic activity of microbial biomass under different long-term management systems. Moreover , the activity ofboth enzymes was comparatively easy to measure. Acid phosphatase and protease activity showed inconsistent responses across a range of soil management practices and had only poor associations with major ecological soil processes. This was more so for acid phosphatase than for pro,tease. Thus, both acid phosphatase and protease were not considered sensitive indicators for soil quality evaluations under the prevailing conditions of long-term management trials at lIT A, Nigeria. Depth or soil sampling ror microbial biomass content as well as for alkaline phosphatase and ß-glucosidase activity should be restricted to 0-5 cm depth, as the most pronounced dirrerences occurred in the top layer due to more favorable environmenta] conditions (substrate availability, moisture regime) under these no-tillage managements. Time of sampling ror microbial biomass should be restricted to the wet season, whereas either enzyme is not necessarily restricted to any particular season. Appropriate indicators should be measurable against some definable local standard. Therefore, a range of threshold values was proposed as the boundary or cut-off between sustainable and unsustainable is more gradual than clear-cut. The lowest limit or baseline for the proposed microbiological indicators could be derived from continuous cropping control treatments of these long-term experiments. Comparison of agricultural land management practices with undisturbed secondary forests as single upper-range standard is not considered suitable, as crop management systems can not have the closed nutrient cycles of undisturbed forests. Hence, appropriate upper-range standards for non-degraded sites could be derived from puerariain situ live mulch at WB 3 site, and for degraded sites from senna at WB I. Pueraria, thus, worked on WB 3, the non degraded site but not on WB I, the severely degraded site, suggesting that pueraria is a maintenance crop. I n contrast, senna worked on the degraded sites and more so on WB I than on D 2. Apparently, senna may work as a restoration crop.