Smriti Limbu, M.Sc. Student, Soil Science, NCSU, will present an exit seminar titled:

“Fall Stockpiled Tall Fescue Response to Nitrogen Fertilization as Affected by Soil Biological Quality“    – under the direction of Dr. Alan Franzluebbers

on 9 August 2017 at 4:00 PM in the McKimmon Room (2223 WMS).

All are welcome to attend.

Abstract

Application of nitrogen (N) fertilization prior to stockpiling is a common practice to promote herbage accumulation. However, there are no soil tests to accurately predict the likelihood of yield response of tall fescue to N fertilization. Soil biological activity contributes to N mineralization, which could be a source of N to plants during the growing season. Pastures with a diversity of management (22 in 2015 and 35 in 2016) were tested in North Carolina, Virginia, West Virginia, and Georgia. Each field had four N treatments (0, 45, 90, 135 kg N ha^-1) replicated four times in a randomized complete block design (16 plots, 3×6 m each). Urea fertilizer was applied at the beginning of September 2015 and 2016. Forage was allowed to grow until harsh winter conditions set in (December to January), at which time plots were mechanically harvested to determine dry matter (DM) production. Harvest was >10 cm in 2015 and >10 cm and 5-10 cm in 2016. Soil was sampled at 0-10 cm depth in August/September of each year. Soil N supplying capacity was assessed with net N mineralization (i.e. inorganic N accumulation) during aerobic incubation at 25⁰C for 24 days. Other soil biological assessments were total N and carbon (C), soil microbial biomass C (SMBC), particulate organic C and N, and the flush of CO_2. Soil N supplying capacity was also assessed with greenhouse growth trials of sorghum-sudangrass during 6 weeks. Two separate greenhouse trials were conducted in June/July 2016 and February/March 2017. Relative yield response at threshold yield efficiency of 5 kg DM kg^-1N was highly correlated with the flush of CO₂ (r² = 0.86), cumulative C mineralization (r2 = 0.76), net N mineralization (r² = 0.80), and SMBC(r² = 0.60), as well as at threshold yield efficiency of 10 kg DMkg^-1 N (r² = 0.86, 0.73, 0.81 and 0.59, respectively). Similarly, the flush of CO_2, cumulative C, net N mineralization, and SMBC were highly correlated with economically optimum N rate at threshold yield efficiencies of 5 kg DM per kg^-1N (r² = 0.63, 0.53, 0.60, and 0.53 respectively) and 10 kg DM kg^-1 N (r² = 0.74, 0.63, 0.70, and 0.53 respectively). There was a weaker correlation between soil biological quality measures and yield response at harvest height of 5-10 cm.  Flush of CO₂was determined to be the best predictor of yield response of stockpiled tall fescue to N fertilization. Crude protein and moisture content of stockpiled tall fescue was significantly related with N fertilizer rate, but not to soil biological properties. Both crude protein and moisture content were greater for >10 cm harvest height than those for 5-10 cm height. In greenhouse growth assays, plant DM correlated strongly with the flush of CO₂, cumulative C mineralization, net N mineralization, and SMB Cin Trial 1 (r²= 0.66, 0.59, 0.62, and 0.58, respectively) and Trial 2 (r² = 0.56, 0.63, 0.64, and 0.55, respectively). Similarly, plant N uptake also correlated with the flush of CO_2, cumulative C mineralization, net N mineralization, and SMBC in Trial 1 (r² = 0.77, 0.72, 0.58, and 0.70, respectively) and Trial 2 (r² = 0.72, 0.81, 0.68, and 0.37, respectively). The flush of CO₂ was the best predictor of plant N uptake in both trials and of plant DM in Trial 1. In Trial 2, net N mineralization was the best predictor of plant DM. Clearly, soil biological activity influences yield response of fall stockpiled tall fescue to N fertilization. Various indicators of soil biological activity (including the short-term assay of the flush of CO₂) were excellent predictors of plant DM production and N uptake in a semi-controlled environment.