Modelling the effects of moisture on barley straw and root decomposition in the field

In a field experiment with barley on a clay soil, four treatments were compared: the control (C) was not nitrogen fertilized or irrigated; drought treatment (D) was identical to C, except that screens had been installed to divert rainwater; irrigation (I) and irrigation-fertilization (IF) treatments were irrigated using a drip-tube system, with liquid fertilizer (260kg N ha −1yr −1) applied daily in IF according to predicted plant uptake. Litter-bags with barley straw were buried in all treatments at 10 cm depth on 19 May 1988 and sampled five times during the growing season. Decomposition rates were calculated assuming exponential decay. The rates were highly dependent on soil moisture, and the constants ranged from 0.54% day −1 in D to 0.92% day −1 in IF. A simulation model with driving variables based on Q 10 temperature dependence and a log-linear relationship between soil water tension and activity was fitted to the same data set. The rate constants became quite similar between the four treatments, ranging from 1.1 to 1.3% optimum day −1. Thus it was concluded that the assumptions and parameters used for temperature and moisture dependence were valid. Roots collected from each of the four treatments on four occasions were put in litter-bags, buried in the soil and taken up after about 20 days. Observed root decomposition rates were lowest in D. The rates for all treatments were highest early in the growing season and decreased thereafter. When temperature and moisture effects were compensated for using the climatic correction factors for barley straw, the decreasing trend remained, which was attributed to reduced decomposability of the roots during the growing season. The results indicate that using a constant value for root decomposability when calculating root turnover can give misleading results.