Time series analysis and mixed models for studying the dynamics of net N mineralization in a soil catena at Gondelsheim (S–W Germany)

Net N mineralization rate having very high annual dynamics is one of the basic parameters characterizing the N balance of the soil. The dynamics of the net N mineralization rates, and the effects of soil temperature and moisture on them, were analyzed for four soils: Eroded Haplic Luvisol, Calcaric Regosol, Gleyic–Calcaric Regosol and Eutric Cambisol located at different catena positions in the western part of the Kraichgau (Germany). The net N mineralization rates were measured in situ on unfertilized and N fertilized plots twice a month over a period of 6 years. Statistical analyses were conducted by a combination of the classical Time Series Analysis, particularly Census-I-Decomposition, with repeated-measures ANOVA using PROC MIXED of SAS. These statistical approaches allow to evaluate the seasonal, trend–cyclic and random components of the time series. In addition to the dynamics of soil temperature, soil moisture and net N mineralization rate, the following main characteristics were investigated: least square means of the investigated data series, their standard errors, and seasonal (annual) components of the series. The amplitudes of annual components of the time series were calculated by two methods: 1) as a half of the difference between maximal and minimal values of the seasonal components determined by Time Series Analysis and 2) based on regression coefficients of a cosine-function with an annual period estimated by the Fourier spectral analysis within the Linear Mixed Models approach. Repeated-measures ANOVA showed significant effects of soil type and location of the soils within the catena on temperature and moisture in 30 cm soil depth. The maximal amplitudes of annual variation of soil temperature and moisture were observed for Calcaric Regosol. This soil was characterized by a maximally pronounced annual cycle of net N mineralization rate, as well as by an increase of mineralization rate after fertilizer application. The annual cycle of net N mineralization rate in Calcaric Regosol was mainly determined by changes of soil temperature, not by the absolute temperature values. However, the long-term trend of net N mineralization rate in Calcaric Regosol followed the trend of soil moisture. The net N mineralization rate and subsequently the total amount of mineralized N increased in the following sequence: Eutric Cambisol < Eroded Haplic Luvisol < Gleyic–Calcaric Regosol < Calcaric Regosol.