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Background: The application of organic matter with a high C/N ratio is effective for the prevention of soil degradation, although this can cause nitrogen starvation. However, some fields are highly productive under nitrogen-starvation conditions. The underlying mechanisms for this are unclear but the correlation between soil microbial biomass (SMB) and crop yield suggests that nitrogen flows from SMB to crops. We aimed to clarify this flow and the source of nitrogen.  Methods: We achieved nitrogen starvation conditions by applying waste mushroom bed and repeated lettuce cropping with different crop management practices, such as watering and fertilizer application. We analyzed correlations among crop yield, SMB, and total nitrogen.  Results: The order of the lettuce yield stably corresponded with the management practice used. The SMB increased remarkably by the time of the second lettuce cropping and showed a strong correlation with crop yield. The nitrogen from the waste mushroom bed was lost by denitrification. The rate of decomposition showed no correlation with yield or SMB.  Conclusions/Discussions: The crop yield corresponded with the management practice earlier than SMB. Namely, no nitrogen flow from SMB affected the crops. Furthermore, most applied nitrogen was denitrified, so the nitrogen flows of applied organic matter, SMB, and crops are independent. Therefore, the nitrogen source of both SMB and crops is biological fixation. The correlation between SMB and crop yield is not a causal relationship. The nitrogen source for both is nitrogen fixation. The application of organic matter enhances this by occurring nitrogen starvation but not providing a nitrogen source.

Crop production under nitrogen starvation conditions: relationships with applied organic matter and soil microbial biomass