Relationship between crop rotation and microbial biomass and activity including field CO2 and N2O fluxes and N mineralization

The impacts of crop rotation and input of organic matter in the form of straw residues and incorporation of catch crops on crop yield, nitrogen uptake and microbial biomass and activity was studied in a field experiment initiated in 1996 at Research Centre Foulum. The parameters studied included substrate-induced respiration (SIR) from where microbial biomass C was calculated, hydrolysis of fluorescein diacetate (FDA), arylsulfatase activity (ASA), N mineralisation, N2O emission, and soil respiration. These experiments were carried out in bare soil plots to measure the effects of previous crops and input of organic matter. In 2001 measurements were carried out in four treatments where the crops in 2000 were winter wheat (W-w), pea-barley (P-b), grass-clover (Gr-cl), and pea-barley undersown with ryegrass (P-b/gr). In the first two treatments straw residues were removed after harvest and only the stubble was incorporated, representing low input of organic matter. In the other two treatments, representing high level of organic matter input, mowed plant material in Gr-cl and ryegrass residues in P-b/gr were left on the ground and incorporated into the soil before sowing the next crop. The development of the 2001-crops was followed in neighbouring non-fertilised plots by measuring the aboveground biomass and nitrogen uptake six times during the growing season. Generally, the treatments with low input of organic matter gave lowest scores for all measured parameters, and W-w was significantly lowest except for soil respiration and N2O emission, which showed high spatial variation. During the period from March to August 78, 100, 105 and 103 kg N ha-1 was mineralised in the treatments W-w, P-b, G-cl and P-b/g, respectively. In comparison, the N uptake in the corresponding 2001-crops was 60, 62, 104, and 89 kg N ha-1. Total soil respiration during the period from April to September was 4.2, 5.3, 5.0 and 5.2 tons C ha-1 in the same treatments, and total N2O emission during the period from ultimo April to ultimo August was 2.6, 2.6, 2.7 and 3.3 kg N2O-N ha-1. The N2O emission factors, which were calculated on the basis of mineralised N, were 0.028, 0.019, 0.019 and 0.028 in W-w, P-b, Gr-cl and P-b/gr, respectively. On average, over the period from April to October 2001 the content of microbial biomass carbon was 230, 305, 305 and 304 µg C g-1 soil, FDA was 89, 90, 120 and 107 µg flourescein g-1 soil, and ASA 22, 24, 31 and 27 µg NP g-1 soil in the four treatments, respectively. A multivariate analysis on all data showed that only the winter wheat with low input of organic matter differed significantly from the other three treatments, whereas the pea-barley was similar to the treatments receiving high input of organic matter. It is assumed that the peas contributed to the higher microbial activity encountered in the subsequent year