Oxygen supply to plant roots in cultivated mineral soils

Roots of all plant species need oxygen for growth, and most crops are supplied with oxygen mainly from soil air. Poor aeration is generally a problem only in wet soils. In compacted soils both large mechanical impedance and poor aeration inhibit the growth of roots and may lead to yield reductions. In this study, the variation in soil air composition, N2O emission and several environmental variables were monitored in loamy clay and loam fields under various combinations of agricultural management practices including compaction, heavy irrigation or wetting, cropping and N fertilization. The aim was to explore the instances in which poor aeration might be detrimental to plant production or environment. More detailed research into the conditions leading to hypoxia in soil and the effects of hypoxia and compaction on soil respiration and barley growth were carried out in pot experiments with fine sand soil treated with combinations of compaction (loose, compacted), wetting (moist, wet) and forced aeration (0%, 2%, 6%, 10%, 20% O2 in N2 gas). Both uncropped and cropped pots were included. In addition, the incidence of wet growing seasons was estimated at five locations in different parts of Finland using long-term weather data to calculate the probability of a precipitation deficit of less than zero at certain times in a growing season. The weather data indicated that wet weather during the early growing season and during harvesting is not uncommon in Finland. The pot experiments showed that O2 concentrations as low as 6% in soil air are adequate for respiration and growth of barley roots in soil at moderate moisture contents, but in wet soil plant growth is impaired even if the soil is aerated with a gas stream containing 20% O2. The observed effects of O2 concentration on the respiration and growth of barley roots compared fairly well with the critical oxygen concentrations calculated by a simple multicylindrical diffusion model, in which the water-film thickness around the roots was estimated using soil water retention data. The experiments indicated that, if the soil is wet, in the conditions prevailing in Finland, hypoxia occasionally limits root growth especially below the ploughed layer. In particular, heavy compaction of clay impairs subsoil aeration for several years. Poor aeration can contribute to a low yield and nitrogen uptake in compacted and wet soils. In wet soil, denitrification is obvious and probably contributes to the yield loss. However, if hypoxia lasts only a short time, the risk to yield loss is small. Also wellaerated soil layers near the surface may prevent yield reduction. The field experiments also showed that leaving land fallow is detrimental to the environment, because it causes N2O emissions. So do irrigation and N fertilization, but cropping is efficient in diminishing the emissions. Large emissions or concentrations of N2O in soil air indicate soil hypoxia, but the importance of this on plant growth is confounded by the heterogeneity of field soils.