Elly Møller Hansen

Nitrate leaching, yields and carbon sequestration after noninversion tillage, catch crops, and straw retention.

Crop management factors, such as tillage, rotation, and straw retention, need to be long-term to allow conclusions on effects on crop yields, nitrate leaching, and carbon sequestration. In 2002, two field experiments, each including four cash crop rotations, were established on soils with 9 and 15% clay, under temperate, coastal climate conditions. Direct drilling and harrowing to two different depths were compared to plowing with respect to yield, nitrate N leaching, and carbon sequestration. For comparison of yields across rotations, grain and seed dry matter yields for each crop were converted to grain equivalents (GE). Leaching was compared to yields by calculating yield-scaled leaching (YSL, g N kg GE), and N balances were calculated as the N input in manure minus the N output in products removed from the fields. Direct drilling reduced yields, but no effect on leaching was found. Straw retention did not significantly increase yields, nor did it reduce leaching, while fodder radish ( L.) as a catch crop was capable of reducing nitrate leaching to a low level. Thus, YSL of winter wheat ( L.) was higher than for spring barley ( L.) grown after fodder radish due to the efficient catch crop. Soil organic carbon (SOC) did not increase significantly after 7 yr of straw incorporation or noninversion tillage. There was no correlation between N balances calculated for each growing season and N leaching measured in the following percolation period.

Biological nitrogen fixation in a grazed perennial grass/clover ley and correlation with herbage and soil variables

Abstract In a grazed grass/clover pasture, biological nitrogen fixation (BNF) is a source of nitrogen (N). The determination of this N input and how it is influenced by N, phosphorus (P) and potassium (K) in the soil, is complicated by grazing animals, which cause variation in defoliation and soil mineral content in both space and time. In a perennial grass/clover ley on a commercial Danish organic farm, exclusively grazed by dairy cows, we investigated BNF, clover proportion of the herbage, accumulated dry matter of clover and N, P and K concentrations in separates of clover and grass in relation to inorganic concentrations of N, P and K in the soil. The 15 N natural abundance technique was used to determine the proportion of clover N derived from the atmosphere (pNdfa) using grass as a non-fixing reference. In September 1997, soil and biomass sampling was carried out on 81 randomly selected plots. The value of pNdfa was on average 0.6, ranging from 0.12 to 0.96 and BNF exhibited a significant but weak correlation ( r =0.432) with accumulation of dry matter in clover. For the significant correlations, r varied between −0.403 and 0.648. The proportion of clover in the sampled herbage averaged 48%, ranging between 18 and 78%, but varied independently of BNF. Due to a high concentration of P and moderate to high concentration of K in the soil, clover and grass were well supplied with P and K, which was confirmed by the concentrations of K and P in shoot material.