title: Productivity depends more on the rate than the frequency of N addition in a temperate grassland.
abstract: Nitrogen (N) is a key limiting resource for aboveground net primary productivity (ANPP) in diverse terrestrial ecosystems. The relative roles of the rate and frequency (additions yr(-1)) of N application in stimulating ANPP at both the community- and species-levels are largely unknown. By independently manipulating the rate and frequency of N input, with nine rates (from 0 to 50 g N m(-2) year(-1)) crossed with two frequencies (twice year(-1) or monthly) in a temperate steppe of northern China across 2008-2013, we found that N addition increased community ANPP, and had positive, negative, or neutral effects for individual species. There were similar ANPP responses at the community- or species-level when a particular annual amount of N was added either twice year(-1) or monthly. The community ANPP was less sensitive to soil ammonium at lower frequency of N addition. ANPP responses to N addition were positively correlated with annual precipitation. Our results suggest that, over a five-year period, there will be similar ANPP responses to a given annual N input that occurs either frequently in small amounts, as from N deposition, or that occur infrequently in larger amounts, as from application of N fertilizers.
authors.affiliation: State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China.
authors.forename: Yunhai
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: 1] State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China [2] Institute of Desertification Studies, Chinese Academy of Forestry, Beijing 100091, China.
authors.forename: Jinchao
authors.initials: J
authors.lastname: Feng
authors.affiliation: Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN 55108, USA.
authors.forename: Forest
authors.initials: F
authors.lastname: Isbell
authors.affiliation: State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China.
authors.forename: Xiaotao
authors.initials: X
authors.lastname: Lü
authors.affiliation: 1] State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China [2] State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China.
authors.forename: Xingguo
authors.initials: X
authors.lastname: Han