Effects of Fumigation on Nitrogen Response and Soil Microbial Activity in Russet Burbank Potatoes

Fumigation is commonly used by potato growers to control soil-borne pathogens. Its short-term benefits include improved disease control and healthier root systems, which may decrease nutrient input requirements. However, fumigation also eliminates beneficial soil organisms, which may depress the soil community’s capacity for pathogen control and nutrient cycling. The goal of our research was to determine the interactive effects of fumigation and N application rate on soil microbial respiration and mineral N concentrations and Russet Burbank leaf greenness and tuber yield, size, quality, sucrose and glucose concentrations, and frying quality. We applied treatments in a split-plot randomized complete block design with four blocks. Whole plots received either Chloropicrin, Vapam, or no fumigant, and each whole plot was split into subplots, each receiving N at one of five total rates (including 40 lbs·ac-1 N as DAP at planting): (1) 40 lbs·ac-1 , 120 lbs·ac-1 , 180 lbs·ac-1 , 240 lbs·ac-1 , and 300 lbs·ac-1 . Fumigation treatments were applied in October and November 2015, and N treatments were applied at shoot emergence in 2016. Soil 24-hour CO2 production, NH4-N, and NO3-N were determined for six-inch soil samples collected before fumigation in 2015 and before planting, during the growing season, and after harvest in 2016. Leaflet SPAD readings were taken at five times between hilling and harvest to measure leaf greenness. Tuber yield, size, quality, sugar concentrations, and frying quality were determined after harvest. Soil from the fumigated plots showed low rates of microbial respiration compared to the non-fumigated plots during the growing season but recovered to non-fumigated levels by harvest. The fumigated plots had elevated NH4-N concentrations before planting, and the plots fumigated with Chloropicrin had high NH4-N and low NO3-N relative to the non-fumigated plots, indicating that fumigation may interfere with nitrification. Leaflet SPAD increased with N application rate but did not respond to fumigation treatment. Total and marketable yields were higher, in the fumigated plots than in the non-fumigated plots, but did not plateau at lower N rates. However, the percentage of yield represented by tubers weighing over six ounces was higher and plateaued at a lower N rate in fumigated plots than in non-fumigated plots, suggesting that fumigation may decrease N requirements for tuber bulking but not for tuber yield. Tuber quality was not meaningfully related to fumigation treatment. The same was true of tuber sucrose and glucose concentrations and French fry reflectance in both the stem ends and bud ends of tubers. Stem-end sucrose concentration and the glucose concentration in both ends of the tuber decreased with increasing N application rate. Bud-end sucrose concentration and French fry reflectance increased with N rate, except that reflectance was relatively high for stem-end French fries from tubers grown at the lowest N rate. Overall, we found that while fumigation increased marketable yield at all N rates tested and decreased N requirements for tuber bulking, it lowered soil microbial activity/diversity during the growing season. Microbial activity was low in all treatments at harvest suggesting that soil improvement practices should be considered following a potato crop.