Assessing the effects of conservation practices and fertilizer application methods on nitrogen and phosphorus losses from farm fields – a meta analysis

Authors: NUMMER, STEPHANIE - University Of Toledo; QIAN, SONG - University Of Toledo; Harmel, Daren

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2018
Publication Date: 8/16/2018
Citation: Nummer, S.A., Qian, S.S., Harmel, R.D. 2018. Assessing the effects of conservation practices and fertilizer application methods on nitrogen and phosphorus losses from farm fields – a meta analysis. Journal of Environmental Quality. 47(5):1172-1178. https://doi.org/10.2134/jeq2018.01.0036.
DOI: https://doi.org/10.2134/jeq2018.01.0036

Interpretive Summary: Conservation practices have been widely used to reduce the quantity of N and P leaving farm fields and address the impact of these nutrients on water quality, but there is a lack of research on the effectiveness of these practices using field scale data. The objective of this study is to quantify the effects of conservation practices on N and P runoff in farmlands. A review of published data called a meta-analysis was conducted using the Measured Annual Nutrient loads from AGricultural Environments (MANAGE) database created by the USDA-ARS. Two different statistical methods (propensity score matching and multilevel modeling) were used to remove the influence of factors that can affect the results. As there is no single best method for addressing the influence of these confounding factors, using two alternative methods is ideal because similar results from both methods increase confidence in findings. Propensity score matching showed that conservation practices can reduce loadings of total P, particulate P, and particulate N by an average of 68%, 83%, and 67%, respectively. The same rate of reduction in nutrient loadings estimated from using multilevel modeling are 58%, 76%, and 64%. While the propensity score method can only yield a mean rate of reduction, multilevel modeling can further estimate the reduction for different crops and fertilizer application methods. The multilevel models showed that conservation practices had the most impact on row crops (e.g. corn and soybeans) and on fields fertilized with injection or surface application.

Technical Abstract: Conservation practices have been widely used to reduce the quantity of N and P leaving farm fields and address the impact of these nutrients on water quality, but there is a lack of research on the effectiveness of these practices using field scale data. The objective of this study is to quantify the effects of conservation practices on N and P runoff in farmlands. A meta-analysis was conducted using the Measured Annual Nutrient loads from AGricultural Environments (MANAGE) database created by the USDA-ARS. Propensity score matching and multilevel modeling were used to remove the influence of confounding factors that results from the observational nature of the database. As there is no single best method for addressing the influence of confounding factors, using two alternative methods is ideal because similar results from both methods increase confidence in findings. Propensity score matching showed that conservation practices can reduce loadings of total P, particulate P, and particulate N by an average of 68%, 83%, and 67%, respectively. The same rate of reduction in nutrient loadings estimated from using multilevel modeling are 58%, 76%, and 64%. While the propensity score method can only yield a mean rate of reduction, multilevel modeling can further estimate the reduction for different sub-groups (such as different crops and different fertilizer application methods) when such grouping is feasible. The multilevel models showed that conservation practices had the most impact on row crops (e.g. corn and soybeans) and on fields fertilized with injection or surface application.