Biological and biochemical quality of pastoral soils: spatial and temporal variability

This paper reports results of the first year of soil biochemical and microbiological monitoring programme carried out to establish "normal" ranges of values for these soil attributes. Study was conducted on 24 farm sites on yellow-brown loam soils around the Waikato area. Twelve dairy farms and a similar number of sheep-beef farms were selected on the basis of high productivity. Soil samples (0-75 mm depth) were collected at 3- monthly intervals and the following measurements were carried out: soil microbial- C, N, S and P, CO2 evolution, substrate-induced respiration, anaerobic mineralisable N, dehydrogenase activity, fluorescein diacetate (FDA) hydrolysis, amounts of soluble-C and N, extractable NO3 and NH4, soil pH, Olsen P, KH2PO4 extractable SO4-S and organic S, and hydraulic conductivity. Climatic data, records of fertiliser and other additives and productivity were also collected to interpret the variations in these properties. Variables measured from the Horotiu and Tirau silt loam soils showed considerable similarity, however, Otorohanga soils had significantly higher amounts of total and extractable soil C and N. As expected, being a higher input system, soil nutrient status (P, SO4, NO3 and NH4) on dairy farms was generally higher than the sheep-beef farms. The most significant difference was for the Olsen P values, which were about 60-70% higher under dairying. Soil pH on dairy farms was significantly higher than sheep- beef farms. However, total C and N values were significantly higher under sheep-beef than dairy farms. Similarly, the amounts of mineralisable N in all seasons were much higher for the sheep-beef than dairy farms. Apart from total microbial S, none of the other microbial biomass measurements showed any significant effect of season or difference among the soil types. This lack of seasonal effect on microbial biomass can be attributed to the unusual mild seasonal variation during the study. For the various microbial biomass measurements, sheep-beef farms generally had significantly higher values than dairy farms. Microbial C, N, SO4 and total S values were significantly higher for sheep-beef than dairying. The ratios between soil C, N to microbial C, N and microbial C:N showed no consistent pattern between the farm types. Keywords: C and N, enzyme activity, microbial biomass, seasonal variations, soil fertility