Factoring P Into N-Based Controlled Drainage Management.

Phosphorus (P) loss from agricultural land means economic loss for producers and may also contribute to water quality impairments. Research was initiated to investigate controlled drainage (CD) as a possible strategy to minimize P loss through artificial subsurface drainage systems to surface water. Controlled drainage, as opposed to conventional free drainage (FD), creates an opportunity to manage drainage systems according to seasonal needs. The objective of this research was to quantify the effectiveness of CD to reduce P loads in Minnesota, USA. A field study was conducted from 2006-2010 on an artificially drained Havelock clay loam soil (mesic Cumulic Endoaquolls). The field site consisted of two independently drained management zones, 15 and 22 ha, respectively. Each zone was outleted into a water table management structure equipped with a stilling well to measure drain outflow volume and P yield and flow weighted mean P concentration (FWMPC). A combination of grab and storm activated discrete samples were collected from each management zone. Samples were analyzed for total phosphorus (TP) and dissolved molybdate reactive phosphorus (DMRP). Additional speciation of TP was conducted in order to explain possible mechanisms of P loss. The experimental approach consisted of an unreplicated, paired-design experiment. Treatment effects were evaluated between the paired sites using a beta distribution after calculating the daily ratio between treatments of interest, e.g. CD/FD for TP yield. Statistical analysis showed that CD effectively reduced annual TP and DRMP yield compared to a FD system but resulted in increased FWMPC for TP. Although CD has a potential agronomic yield advantage and is a valuable system for controlling NO3-N loss through artificial subsurface drainage, it could constitute a problem with regard to P export to surface water.