NO-TILL CORN AND PHOSPHORUS
In the June and July issues of the Farm Report I discussed the hydrology and nitrogen (N) dynamics from a recent no-till study that was completed at Miner Institute. We’ll finish up the discussion of this study by delving into the impacts of no-till on phosphorus (P) losses from the field.
The majority of losses occurred via tile drainage as it dominated the water budget of both fields, with 73% and 59% of total P losses occurring from tile drainage. However, the surface drainage showed a higher capacity to produce total P losses as tile drainage represented a minimum of 88% of the water budget in NT and a maximum of 95% of the water budget from TILL. If the tile and surface waters were equally capable of generating P losses, the amount of P lost from each source would be equivalent to its portion of the water budget.
Field-scale total P annual loads ranged from 0.05-1.84 lb/acre for the TILL field and from 0.12 – 1.23 lb/acre for the NT field. The field-scale annual soluble reactive P (SRP) loads ranged from 0.03 – 1.31 lb/acre and 0.06 – 0.81 lb/acre for the TILL and NT fields, respectively. Mean annual SRP and total P loads are summarized by period in Figure 1.
Unlike what was observed with N losses, there was no obvious pattern evident in the tile P losses, particularly with respect to cumulative flows, as seen in Figure 2. Total P losses appeared to be relatively independent of total flow, indicating that losses were primarily source-limited rather than transport-limited. Conversely, total P losses in surface runoff occurred relatively consistently so that the slopes of the lines were relatively consistent between NT and TILL. However, total P losses in surface runoff were substantially lower overall in TILL but were closely proportional to the amount of surface runoff that occurred in each of the fields.
The no-till treatment ultimately increased the SRP and total P losses in tile drainage which resulted in an increase at the field-scale (surface + tile) as well, with a 206% increase in the SRP load observed at the field-scale. Similarly, we also observed a 125% increase in total P loading due to the no-till treatment. The increase in both SRP and total P is likely due to the influence of preferential flow pathways (PFPs), or macropores, developing in NT during the treatment period when tillage was eliminated and soil structure improved, rapidly delivering surface waters to the tiles with little to no interaction with the soil and its physicochemical filtration capacity. Although we did not attempt to quantify the presence and extent of these PFPs, the increased rate of tile drainage during the treatment period of NT indicates a strong likelihood that these pathways developed, were active, and influenced nutrient transport dynamics.
Related to the development of PFPs, the other factor that likely influenced P transport was the 35% reduction that was observed in TSS losses from NT. As PFPs develop and begin to transport a larger fraction of the total drainage water from a field, there is typically a concomitant reduction in surface runoff and erosion. As the observed 17% increase in tile drainage flows from the no-till treatment likely resulted in a reduction in surface runoff, there was also a decrease in the amount of erosion. As erosion and TSS losses decrease, there is often an increase in the ratio of SRP to total P due to the loss of the particulate (nonreactive) P fraction. As discussed above, this was observed with field-scale SRP losses that were nearly twice the magnitude of total P losses, along with the reduction of TSS losses in NT.
So ultimately we did not see the results we were hoping for, which would have been a reduction in SRP and total P loading as a result of the no-till treatment. However, we shouldn’t use this study as evidence that no-till is an ineffective BMP. Rather, we should recognize that, at least in some cases, it may not be having its intended benefits for water quality. However, the improved soil structure, enhanced drainage abilities, increased biodiversity and biomass are all examples of soil quality measures that have been well-studied and oft-proven to be enhanced when no-till is utilized. Thus, we should not abandon this practice, but rather recognize its limitations and pair it with additional practices that can help address those, including, but not limited, to cover cropping, manure injection, and drainage water management.
— Laura Klaiber
