Nutrient Dynamics in Soil and Water
Researchers involved:
Mario Tenuta
Don Flaten
Graham Phipps
Denis Tremorin
Luca Coppi
Siobhan Stewart
Background
Nutrients in hog manure are a valuable resource for crop and forage land. Managing nutrients in manure or any fertilizer in a sustainable manner is of utmost importance, since lost nutrients mean lost productivity and potential contamination of ground and surface water with nitrates and phosphates.
Although manure is generally applied as a nitrogen (N) source, there is growing concern regarding soil phosphorus (P) levels and contamination of waterways and lakes with phosphates. The P removal rate by crops is much lower than the N removal rate. Hog manure can have more P than N required by crops. When manure is applied at rates to meet crop N demand, accumulation of soil P can result. This P that is left over in the soil may, over time, build up to the point of becoming a pollutant.
Nutrient dynamics in a forage-based manure management system are an important component of the sustainability of the system. For this reason, studying nutrient dynamics is a major part of the La Broquerie Research Project.
Objectives of the nutrient dynamic study were:
- to assess residual nutrient levels
- to monitor the buildup of P
- to study the spatial variability of nutrients in a pasture system, particularly in bare earth patches around livestock waterers and
- to measure the distribution of nutrients in the soil profile and in groundwater
Methods
Two types of soil samples were taken in all plots each fall prior to the fall manure application:
- Soil profile samples were taken at one-foot intervals to a depth of 4 feet using a hydraulic auger. These samples were analyzed for available plant P (modified Kelowna method in all years and Olsen-P method in 2006 and 2007), available plant N (ammonium and nitrate), and chloride (used as a tracer to detect potential for nutrient movement through the soil profile).
- Plant available nutrient samples were also taken each fall to depths of 0-2 inches and 2-12 inches using a Dutch auger. These samples were analyzed for plant available N and P, as well as chloride, total organic carbon, total nitrogen and total phosphorus.
Groundwater flow at the research site is toward the east-north-east. Therefore, monitoring wells were installed just outside the west side of the research area (background wells) and on the down-gradient (north-east) side of all plots. Shallow groundwater samples were taken from monitoring wells located throughout the research area and were analyzed for nitrate, phosphorus and chloride content. Water table depth was also measured.
Soil in and around bare earth areas surrounding livestock waterers and mineral supplements was sampled at depths of 0-2 inches and 2-12 inches and analyzed for plant available N and P and chloride. Samples were taken in the bare earth area, the transition zone, the grassed fringe of the bare earth area, and the grassed plot.
Results
Soil Nutrients
At the beginning of the project, soil nutrient levels were low in all plots and were similar between treatments. N and P levels were somewhat higher at the soil surface than deep in the soil profile due to the accumulation of nutrients in rooting zone associated with plant roots and organic matter.
Manure application had little effect on soil ammonium (NH4+) content. Nitrate levels increased slightly in manured plots, but only in the first foot of the soil profile, indicating that nitrate was not moving down through the soil. Plant uptake likely prevented nitrate from moving down below the rooting zone.
Manure application caused an increase in soil P levels in the first two feet of the soil profile, especially near the soil surface. Each successive manure application was accompanied by an increase in soil P levels. The increase was greater in the grazed plots than in the hayed plots, since the phosphorus removal rate was higher in the hay system than in the pasture system.
Olsen-P levels in the upper 6 inches of soil were estimated based on soil test results for 0-2 inch and 2-12 inch depths, in order to put soil P levels into the perspective of current manure management regulations. Under current regulations, manure can be applied according to the N requirements of the following crop as long as soil P remains below 60 mg/kg or parts per million (ppm) in the upper 6 inches of the soil profile. In 2006, estimated Olsen-P reached levels of 30 mg/kg in some treatments (Figure 1). These levels of P accumulation have been verified by sampling to a depth of 6 inches prior to spring manure application in 2007. Based on trends observed so far, soil P could reach 60 mg/kg as early as 2009 at current manure application rates.
Figure 1. Estimated Olsen-P in 2003-2006 in upper 6 inches of soil, based on soil test results at depths of 0-2 and 2-12 inches. Lines connecting the circles indicate trends based on this data.
Chloride is naturally present in manure and serves in this project as a tracer indicating the presence of manure constituents moving down soil profiles. Unlike nutrients in the manure, chloride is not consumed by plants or soil organisms and does not undergo reactions in or sorption to soil. Chloride was the only element in manure that we tracked that seemed to be mobile and move down the soil profile. Levels of chloride in 2004 dramatically increased for several plots receiving manure. Chloride levels also increased at three-foot and four-foot depths for two control plots in 2006, indicating the possibility of lateral movement of chloride from adjacent plots receiving manure or upward movement of chloride from groundwater as evapotranspiration occurred in the very dry year of 2006.
Nutrients in Groundwater
Analysis of shallow groundwater samples indicated that nutrients were present in low concentrations in some wells. The nitrate concentrations of most plots were similar to the background wells outside the research area and were below 1 mg N/L (Table 1). Exceptions were the split - grazed and full - hayed treatments, which contained 1 mg N/L. Nitrate concentrations in all plots were well below the drinking water guidelines of 10 mg N/L. Chloride concentrations appeared to be increasing in the manured plots, as was expected with the addition of chloride in the manure. Differences exist in the phosphorus concentrations of the manured and non-manured plots, however, there are no apparent trends associated with the addition of manure.
| Manure Management | Forage Management | Nitrate (mg N/L) | Chloride (mg/L) | Total P (ug P/L) | Phosphorus CCME Rating* |
|---|---|---|---|---|---|
| Control | Hay | 0 | 1 | 5 | oligotrophic |
| Control | Grazed | 0 | 4 | 5 | oligotrophic |
| Split (Spring/fall) | Hay | 0 | 10 | 2 | ultraoligotrophic |
| Split (Spring/fall) | Grazed | 1 | 15 | 11 | mesotrophic |
| Full (Spring) | Hay | 1 | 5 | 6 | oligotrophic |
| Full (Spring) | Grazed | 0 | 1 | 12 | mesotrophic |
| *Canadian Council of Ministers of the Environment (CCME) rating of nutrient P status of water. Ratings from nutrient poor to nutrient rich are ultraoligotrophic, oligotrophic, mesotrophic, eutrophic. | |||||
Bare Earth Patches
Over the course of the study, bare earth patches developed in high-traffic areas around livestock waterers and mineral supplements in grazed plots. In these areas, deposition of feces and urine is high and plant uptake of nutrients is low or non-existent. Further, soil organic matter and roots decompose in these areas, possibly increasing available nutrients. Therefore, these areas could potentially become point sources of groundwater contamination with nutrients.
Nitrate and phosphorus levels increased dramatically from the grassed plot to the centre of the bare earth patches, particularly near the soil surface (Fig. 2). Particularly phosphorus levels in the bare earth areas were very high for the 0-5 cm depths but with no difference apparent between manure treatments.
Even the control treatment had high nutrient levels in the bare earth patches (Fig. 2), indicating that cattle transported nutrients to those spots from the rest of the pasture. In plots where manure had been applied, nutrient concentrations were even higher, reflecting the higher number of animals in those plots.
Figure 2. Nitrate-N and Olsen-extractable P in the 0-2 inch depth of soil sampled at increasing distnaces from bare earth patches around waterers and mineral supplements (Bare to Grassed Plot) in grazed pasture. Soil analysis done in 2006, after three years of consecutive grazing.
Conclusions
- Analysis of soil samples has shown that chloride is moving down through the soil profile and into groundwater in manured plots.
- Phosphorus concentrations are increasing within the soil surface (root zone) but perhaps not beyond.
- There is no evidence that nitrate or phosphorus is moving into the groundwater.
- Nutrient buildup is evident around mineral feeders and waterers where animals congregate and forage yield is limited. The importance of these bare earth patches is not known but is being examined.