Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2012
Publication Date: N/A
Citation: N/A Interpretive Summary: Nitrogen is an important and often limiting nutrient for plant growth in both natural and agricultural ecosystems. Herbivorous insects may consume and excrete a large proportion of the nitrogen contained in plant tissues, especially during insect oubreaks. The fate of this nitrogen depends in large part on how quickly the nitrogen is released from insect excrement and from the bodies of dead insects. Grasshoppers frequently reach very high densities on grasslands and in agricultural fields, and encounter a wide variety of food items with a wide range of N concentrations. Experiments were conducted to determine the effects of diet quality on availability of N in grasshopper excrement. Results show that diet has a strong influence on how much N is in grasshopper excrement and, in turn, how much of that N will be available for plant uptake. Much of the N in the grasshoppers' excrement will be available for plant growth within the same growing season when grasshoppers feed on plant tissues with more than 2.5% N. This information will increase our understanding of the role herbivorous insects play in nutrient cycles, and the fate of N within agricultural and natural ecosystems.
Technical Abstract: Insect herbivory can produce a pulse of mineral nitrogen (N) in soil from the decomposition of frass and cadavers. In this study we examined how diet quality affects rates of N and carbon (C) mineralization from grasshopper frass and cadavers. Frass was collected from grasshoppers fed natural or meridic diets which varied in N content. Frass was also collected from naturally foraging grasshoppers. Nitrogen concentration of frass was directly proportional to diet N, but N content of cadavers was not affected by diet. Incubations of soil plus frass were performed at constant soil moisture and temperature (15°C) for 28 days, after which levels of mineral N (KCl extract) were determined. About 44% of C and N from the cadavers were mineralized after the 28 d incubation. Carbon mineralization of frass was not affected by diet or frass N, but varied considerably among different food treatments: from 15 to 46% of the carbon in frass was released as CO2. Generally, frass with C:N ratio greater than 20 resulted in net immobilization of N. Results suggest that much of the N in grasshopper frass and cadavers is labile and rapidly available for plants, depending on the quality of food consumed by the grasshoppers.