Total and labile soil organic N as influenced by crop rotations and tillage in Saskatchewan soils
Crop rotations and tillage practices affect quantity and quality of soil organic N (SON). Six field experiments varying from 8 to 25 years across various soil zones in Saskatchewan were conducted to evaluate the impact of crop rotations and tillage practices on SON and mineralizable N. In the Brown soil zone, continuous wheat over the fallow-wheat increased SON from 6 to 17 kg N ha-1yr-1. In the Dark Brown soil zone, continuous cropping increased SON by 30.7 kg N ha-1yr-1, compared with cropping systems containing a fallow once every three years in the Elstow clay loam, and in the Black soil zone by 24 kg N ha-1 yr-1, compared with cropping systems containing a fallow once every four years in the Indian Head clay. Increases in SON under continuous cropping over fallow-containing cropping systems were greater in Dark Brown and Black soil zones than in the Brown soil zone even though the frequency of fallow was less in the Dark Brown and Black soil zones. Increases in SON with continuous cropping also accompanied by a greater increase in the labile fraction of SON in the Brown soil zone, but not in the Dark Brown and Black soil zones. Notillage compared with conventional tillage increased SON by 4.5 kg N ha-1yr-1 for the Hatton fine sandy loam, 3.6 kg N ha-1yr-1 for the Swinton silt loam, 14.6 kg N ha-1yr-1 for the Sceptre clay, 30.5 kg N ha-1yr-1 for the Elstow clay loam, 43.6 kg N ha-1yr-1 for the Melfort silty clay loam, and 34 kg N ha-1yr-1 for the Indian Head clay. Increases in SON under no-till followed by a greater increase in the mineralizable N in the 0-7.5-cm soil for the Hatton fine sandy loam, Melfort silty clay loam and Indian Head clay, indicating that this increased SON under no-till is more labile or mineralizable than the native SON. However, the increased SON under no-till in the Swinton silty loam, Sceptre clay and Elstow clay loam did not translate to any increase in the soil mineralizable N, indicating that this increased SON was more resistant to decomposition than the native SON. Therefore, increases in SON under improved management practices such as conservation tillage and extended crop rotations may not necessarily increase soil N availability.
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