The distribution of net nitrogen mineralisation within surface soil. 2. Factors influencing the distribution of net N mineralisation

This investigation examined the magnitude of gradients in net nitrogen (N) mineralisation through the surface 20 cm of soils used for cropping, and factors influencing these gradients. Soils used were W (Red Kandosol) from a previous field study, CT (conventional tillage) and DD (direct drilled) from a longterm rotation experiment on a Red Kandosol, and YS (Yellow Sodosol) from under wheat established by reduced tillage. Soil was collected in 2-cm intervals to a depth of 10 cm and at 5-cm intervals between depths of 10 and 20 cm. All depths were used from soil W but only depths of 0-2, 8-10, and 15-20 cm were used for the remaining soils. Soils were incubated for 0, 2, 4, 8, and 16 weeks under controlled conditions (soil water, 80% of water-holding capacity; temperature, 20 degrees C; aeration, 1 h/week). Net N mineralisation and carbon dioxide (CO 2) evolution were measured. Heterotrophic activity as measured by CO 2 evolution decreased with depth but was still evident in soil from below 10 cm. This activity was strongly correlated with the concentration of organic carbon (C) (r = 0.88, P < 0.05) and soil pH (r = 0.80, P < 0.05). The percentage of organic C released as CO2 was highly correlated with pH (r = 0.86, P < 0.05), implying that, when sampled at an appropriate scale, acidity limits heterotrophic activity. Net N mineralisation decreased with depth in all soils. In soil W, 59% of net N mineralised originated from 0 to 2 cm, 87% was from the surface 6 cm, and only 1% was from below 10 cm. This was similar to the distribution of net N mineralisation found in the field experiment reported in the previous paper. Net N mineralisation was correlated with CO 2 evolution (r = 0.97, P < 0.01) and pH (r = 0.73, P < 0.05). The C : N ratio increased from 18 in the surface 6 cm to 27 between 10 and 20 cm. The lack of net N mineralisation below 10 cm was attributed to this increase in the C : N ratio. This is the first observation of gradients in net N mineralisation with depth at this scale in different tillage systems.