Dr Tanvir Shahzad¹, Dr Sabir Hussain¹, Dr  Faisal  Mahmood¹

¹Department Of Environmental Sciences & Engineering, Government College University Faisalabad, Faisalabad, Pakistan

Microbial-N mining theory states that if fresh-C is provided under limited N, microbes mine soil organic-matter (SOM) to release mineral-N in order to assimilate the fresh-C thereby leading to increased SOM mineralization. Whereas, in excess N, microbes assimilate the fresh-C using the excess mineral N implying that higher N promotes SOM stabilization. However, it’s unclear whether this theory holds in the presence of plants. We grew L. perenne (Lp) in monocultures with different mineral N supply or in mixed culture with T. repens (Lp+Tr) for N supply via N-fixation. Bare soils were used as controls. Lp were either supplied with high (Lp-N++, 120 kg-N-ha-1) or low N (Lp-N+, 120 kg-N-ha-1) 141 and 342 days after sowing. All the pots were kept under continuous 13C labelling to distinguish soil- (Rs) and plant-derived CO2-C (Rp). The difference of Rs between planted and bare soils were called rhizosphere priming effect (RPE). The Lp-N++ did not differ in terms of RPE with Lp-N+ for most of 504 days except between 289 to 402 days when high-N-induced growth induced higher RPE in Lp-N++. The RPE in Lp + Tr where N was being fixed, remained higher than that in Lp monocultures. The specific RPE (RPE/Rp) was significantly lower in Lp+Tr whereas it was similar for Lp-N++ and Lp-N+ treatments, indicating that the availability of fresh C (high vs low plant growth) controlled the RPE, and not the mineral N. In conclusion, microbial N mining theory doesn’t hold in live soil-plant systems.