Watershed Buffering of Legacy Phosphorus Pressure at a Regional Scale: A Comparison Across Space and Time

Abstract

Phosphorus (P) plays a crucial role in both agricultural productivity and water quality. There has been growing recognition of the importance of 'legacy' P (that which has accumulated in watersheds over time), for understanding contemporary water quality outcomes; however, little is known about how different watersheds respond to cumulative P pressures. The "buffering capacity" concept describes the ability of watersheds to attenuate P loading to surface waters by retaining cumulative P inputs over time. To explore the role of various watershed characteristics in buffering capacity, we compared Net Anthropogenic P Input (NAPI) estimates to riverine total P flux across a thirty-year time span (1981-2011) in 16 large watersheds in southern Quebec, Canada. We used this historic P data to calculate indices describing long-and short-term buffering for these watersheds. We then examined the correlation between this buffering capacity and a set of key geochemical, hydrological, landscape, and socio-ecological variables that were hypothesized to influence P buffering dynamics. Both long-and short-term buffering metrics were most strongly correlated with hydrologic characteristics, indicating that watershed hydrology may be the most prominent characteristic in P buffering within watersheds. However, we found that considering estimates of long-term P accumulation along with biophysical characteristics of the watershed (including hydrology) predicts water quality better (R 2 =0.69) than either factor would alone (R 2 =0.35). Our findings provide a step towards improving models of watersheds' unique relationships to P pressure and can help guide management of historically agricultural landscapes with considerable amounts of legacy P. Manuscript Highlights Watershed buffering is a concept used to explore how watersheds respond to legacy P pressure Hydrology and landscape features play major role in watershed buffering 40 Combining P data and watershed characteristics gives a comprehensive picture of watershed buffering

Decreased buffering capacity and increased recovery time for legacy phosphorus in a typical watershed in eastern China between 1960 and 2010

“…2). These results imply a considerable contribution of legacy P sources as well as an increased fractional export of P inputs to rivers corresponding to decreased buffering capacity over time Sharpley et al 2013;Kusmer et al 2018).…”

“…To examine changes in P buffering capacity for the Yongan watershed over time, this study adopted the short-term buffering index (SBI) and long-term buffering index (LBI) proposed by Kusmer et al (2018). SBI was calculated as the difference between annual NAPI and annual riverine P flux for the corresponding year:…”

“…In comparison, the short-term P buffering index (SBI) decreased more rapidly showing an overall decline of 65% from 0.72 in 1980 to 0.25 in 2010. The SBI value is an indication of the short-term trapping/retention ability of recently applied P within the watershed, while the LBI value reflects the capacity of the watershed to retain historically accumulated surplus P (Kusmer et al 2018). The LBI was higher than SBI ( Fig.…”

“…Previous studies posit that legacy P stocks are mainly regulated by watershed P buffering capacity, i.e., the ability of a watershed to modulate P flux to downstream waters by retaining excess P within watersheds and modulating P release to receiving waters Kusmer et al 2018;Withers et al 2018). P buffering capacity is influenced by soil factors (e.g., soil P content, texture, chemistry and mineralogy, Kleinman et al 2011), hydrological factors (e.g., hydrologic flowpaths, water yield and baseflow index, Kusmer et al 2018), landscape factors (e.g., topography, land use/land cover, Qiu and Turner 2015), and human and management factors (e.g., artificial soil drainage, river channelization and riparian zone management, Reed and Carpenter 2002;Gentry et al 2007). Although watershed P buffering capacity is gaining increased attention in developed nations Gu et al 2019), there is a lack of information concerning P buffering capacity and legacy P dynamics for watersheds in developing nations.…”

“…The Yongan watershed is a typical watershed in eastern China, where watersheds generally have small to medium drainage areas (e.g., 100-10,000 km 2 ) with sub-tropical climate and highly weathered and acidic soils (Oxisols and Ultisols), and have experienced rapid population growth and economic development in recent decades (Chen et al 2015a(Chen et al , 2016. By integrating the net anthropogenic P input approach (Russell et al 2008) with long-and short-term buffering indexes (Kusmer et al 2018), we examined changes in the long-term watershed P buffering capacity and its response to changes in land use/land cover and climate. A new model that incorporates accumulated legacy P stocks and annual precipitation was then developed to estimate the critical legacy P stock threshold necessary to achieve a targeted water quality standard.…”