Yields of wheat and soil carbon and nitrogen contents following long‐term incorporation of barley straw and ryegrass catch crops

Three successive crops of winter wheat were grown on a sandy loam to test the residual effect of long-term annual incorporation of spring barley straw at rates of 0, 4, 8 and 12 t ha -1 , and ryegrass catch crops with or without additions of pig slurry. Soil receiving 4, 8 and 12 t ha -1 of straw annually for 18 years contained 12, 21 and 30% more carbon (C), respectively, than soil with straw removal, and soil C and nitrogen (N) contents increased linearly with straw rate. The soil retained 14% of the straw C and 37% of the straw N. Ryegrass catch-cropping for 10 years also increased soil C and N concentrations, whereas the effect of pig slurry was insignificant. Grain yield in the first wheat crop showed an average dry matter (DM) increase of 0.7 t ha -1 after treatment with 8 and 12 t straw ha -1 . In the two subsequent wheat crops, grain yield increased by 0.2-0.3 t DM ha -1 after 8 and 12 t straw ha -1 . No grain yield increases were found after 4 t straw ha -1 in any of the three years. Previous ryegrass catch crops increased yields of wheat grain, but effects in the third wheat crop were significant only where ryegrass had been combined with pig slurry. Straw incorporation increased the N offtake in the first wheat crop. In the second crop, only 8 and 12 t straw ha -1 improved wheat N offtake, while the N offtake in the third wheat crop was unaffected. Ryegrass catch crops increased N offtake in the first and second wheat crop. Again, a positive effect in the third crop was seen only when ryegrass was combined with slurry. Long-term, annual incorporation of straw and ryegrass catch crops provided a clear and relatively persistent increase in soil organic matter levels, whereas the positive effects on the yield of subsequent wheat crops were modest and transient.

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