Biological and chemical properties of arable soils affected by long-term organic and inorganic fertilizer applications

Using soils from field plots in four different arable crop experiments that have received combinations of manure, Lime and inorganic N, P and K for up to 20 years, the effects of these fertilizers on soil chemical properties and estimates of soil microbial community size and activity were studied. T he Boil pH was increased or unaffected by the addition of organic manure pl us inorganic fertilizers applied in conjunction with lime, but decreased in the absence of liming. The soil C and N contents were greater for all fert ilized treatments compared to the control, yet in all cases the soil sample s from fertilized plots had smaller C:N ratios than soil from the unfertili zed plots. The soil concentrations of all the other inorganic nutrients mea sured were greater following fertilizer applications compared with the unfe rtilized plots, and this effect was most marked for P and K in soils from p lots that had received the largest amounts of these nutrients as fertilizer s. Both biomass C determined by chloroform fumigation and glucose-induced r espiration tended to increase as a result, of manure and inorganic fertiliz er applications, although soils which received the largest additions of ino rganic fertilizers in the absence of lime contained less biomass C than tho se to which lime had been added. Dehydrogenase activity was lower in soils that had received the largest amounts of fertilizers, and was further decre ased in the absence of lime. This suggests that dehydrogenase activity was highly sensitive to the inhibitory effects associated with large fertilizer additions. Potential denitrification and anaerobic respiration determined in one soil were increased by fertilizer application but, as with; both the microbial biomass and dehydrogenase activity, there were significant reduc tions in both N2O and CO2 production in soils which received the largest ad ditions of inorganic fertilizers in the absence of lime. In contrast, the s ize of the denitrifying component of the soil microbial community, as indic ated by denitrifying enzyme activity, was unaffected by the absence of lime at the largest rate of inorganic fertilizer applications. The results indi cated differences in the composition or function of microbial communities i n the soils in response to long-term organic and inorganic fertilization, e specially when the soils were not limited.