Plant root nitrogen (N) uptake and assimilation systems are highly responsive to external N availability. Expression patterns of genes involved in these systems may serve as a sensitive “plant’s eye view” of soil N availability, even in situations when N cycling is so rapid that plant/microbial N uptake and N mineralization are tightly coupled and inorganic N does not accumulate. This would allow better understanding of N mineralization-immobilization dynamics, and thus factors that lead to pulses of N excess and periods of N deficiency in agroecosystems. We explored the relationship between soil biogeochemical processes and dynamic plant nutrient uptake by using novel applications of molecular biology techniques coupled with conventional metrics of soil N availability at an organic farm in the Sacramento Valley, California. Following N treatments (6.5 and 65 µg-NH4+-N g-1 soil) designed to simulate a nutrient patch, we measured changes in expression of tomato (Solanum lycopersicum L.) root genes involved in N uptake and assimilation as well labile soil N pools, bioassays for microbial N transformations, and root/shoot N concentration. Since organic agriculture relies on microbial transformations of soil organic matter to render N available for strong plant demand, it is an excellent system to test these interactions.
Results/Conclusions
Tomato root genes responded rapidly to N additions and to the subsequent changes in soil inorganic N concentrations. The high N treatment significantly increased soil ammonium (NH4+) and nitrate (NO3-) pools after 48 hours and significantly increased expression of an NH4+ transporter, LeAMT2, and glutamine synthetases, LeGS and LeGTS1. These genes also trended toward higher expression levels under the low N treatment, in spite of the lack of a detectable increase in soil inorganic N for this treatment. Root N concentration was significantly higher in both the low and high N treatments relative to the control after 120 hours. Rapid depletion of soil NH4+ after 48 hours indicates high N demand and likely high nitrification rates, since soil NO3- levels remained elevated 120 hours following N treatments. No differences in gene expression were observed 120 hours after the treatments. Soil microbial biomass carbon did not differ among the treatments, suggesting that N was not limiting to microbial abundance.
The high sensitivity of these N uptake and assimilation genes to soil N cycling makes them strong candidates for diagnostic indicators of plant N availability, which could facilitate adaptive nutrient management on organic farms.