Seasonal Phosphorus Losses in Runoff from a Coastal Plain Soil

Abstract

Nutrients from agricultural fields can contaminate runoff and groundwater and enhance eutrophication. Runoff, soil loss, soluble P (SP), and bioavailable P (BP) associated with runoff and soil loss were measured from six 2 h simulated rainfall events (1 in./h) to determine amount of P transported during a corn ( Zea mays L.) growing season and to develop relationships that predict P amounts and forms leaving different size field plots. Two 60‐sq‐ft microplots and two 6500‐sq‐ft mesoplots (3% slope) were located on a Tifton loamy sand (fine‐loamy, siliceous, thermic Plinthic Kandiudult). Phosphorus was broadcast at 31 lb P/acre, then incorporated to a depth of 6 in. Corn (conventional tillage) was planted after fertilizer application. Six simulated rainfall events occurred 5 d before and 1,14, 29, 49, and 108 d after fertilizer application. Total runoff and sediment yield from events 3 to 6 generally decreased with time for all plots. Microplot runoff for each event was greater than that from mesoplots. Microplot soil loss was greater than that from mesoplots for events 3 and 4, but less for events 5 and 6. Runoff from mesoplots ranged from 0.14 to 0.65 in. and total soil loss ranged from 136 to 1097 lb/acre, whereas total runoff from microplots ranged from 0.30 to 0.91 in. and total soil loss ranged from 152 to 2820 lb/acre. For the five events after fertilization, SP losses decreased with days since fertilization, and were greatest 1 d after fertilizer application (0.70–0.99 lb P/acre—129—181 g P/ha). Similar trends were obtained for microplots (event 2 SP losses were 1.21–1.56 lb P/acre—222–287 g P/ha). Total BP E (1.22 lb P/acre—225 g P/ha) and BP (1.51–1.56 lb P/acre‐277–286 g P/ha) losses from mesoplots were greatest 14 d after fertilization. Microplot BP e values decreased with time, and BP values for events 2 to 6 decreased with time. Microplot BP values were greatest for event 2 (1.89–2.64 lb P/acre—347–485 g P/ha). Mesoplot SP losses from each event were related to days since fertilization and runoff volume for each event, whereas total BP E and BP losses for mesoplots were related to days since fertilization and total soil loss. Similar relationships were found for microplots. The P index (PI) was developed to identify site characteristics that may influence P movement. The PI for this site fell into the medium site vulnerability class, and was dominated by the soil P test category. The medium site vulnerability class seemed to underestimate the total BP losses from the six rainfall events. Research Question Nutrients from agricultural fields can contaminate surface and ground‐waters and enhance eutrophication. This study measured runoff, soil loss, and different forms of P associated with runoff and soil loss from six 2 h simulated rainfall events to determine amount of P transported during a corn growing season and to develop relationships between P amounts leaving different size field plots and runoff, soil loss, and days since fertilization. Literature Summary Transport of soluble and sediment‐associated P influences soil fertility strategies and can accelerate eutrophication in lakes and streams surrounding agricultural fields. Variability in amount and form of P in runoff and on sediment occurs with different soil/climate/management conditions. Different amounts and forms of P measured from limited conditions have been reported. Also, predictive equations have been developed from these data. However, experimental conditions from which those data and equations were developed prohibit them from being extended to different “field” scenarios. Study Description Two 60‐sq‐ft microplots and two 6500‐sq‐ft mesoplots (3% slope) were located on a Tifton loamy sand. Phosphorus was broadcast at 31 lb P/acre (71 lb P 2 O 5 /acre), then incorporated to a depth of 6 in. Conventional‐till corn ( Deltapine G4543 ) was planted in the middle of April after fertilizer application. Simulated rainfall (1 in./h) for 2 h events occurred 5 d before and 1, 14, 29, 49, and 108 d after fertilizer application. Runoff, oil loss, soluble P, and bioavailable P, were measured throughout each rainfall event and from each plot type. Applied Questions For the growing season scenario studied, what were the magnitudes and trends of P losses? Soluble P losses were low before P fertilizer application and greatest (0.66–0.99 lb P/acre or 1.5–2.3 lb P 2 O 5 /acre) for the first rainfall event following fertilizer application. After this, soluble P losses declined for the remaining rainfall events. Loss of bioavailable P on sediment was greatest for the event 29 d following fertilization (1.22 lb P/acre or 2.8 lb P 2 O 5 /acre), then losses of bioavailable P on sediment also declined. Total bioavailable P increased for the first three events up to 29 d following fertilization, then declined. Maximum loss of total bioavailable P was 1.54 lb P/acre or 3.5 lb P 2 O 5 /acre. Majority of total bioavailable P losses were associated with sediment. Rates of all forms of P lost within the first event following fertilization increased during the early part of the event, after that soluble P and total bioavailable P either continued to increase or leveled off (Fig. ). Bioavailable P on sediment tended to decrease after this initial increase. Phosphorus totals (sum of all events) for the entire growing season are shown in Table to estimate how much P could be lost under the conditions studied. Total bioavailable P leaving all plots averaged about 5.29 lb P/acre (12 lb P 2 O 5 /acre). This amount of transported P causes concerns about detrimental effects of P Full scientific article from which this summary was written begins on page 507 of this issue. to off‐site areas and the amount of transported P that could have been used for crop production. From the data obtained, can relationships between site characteristics and P losses be developed to predict the amount of P transported from an agricultural environment? The phosphorus index (PI) was developed to identify sites that have potential for P movement, and identify site characteristics that affect P movement from field sites. The PI for this study site was in the medium site vulnerability class, and was dominated by the soil P test category. The PI cannot accurately estimate the form or magnitude of P losses from a specific site given site characteristics, and underestimated total bioavailable P losses added over the six simulated rainfall events. Total P losses were exponentially related to days since fertilization, total runoff from each event, and total soil loss from each event. Relationships between days since fertilization and P losses were significant (r 2 values ranged from 0.67 to 0.96). Total P losses were significantly related to total runoff and soil loss over the corn growing season. Values of r 2 ranged from 0.32 to 0.91. Total Soluble P (SP), bioavailable P attached to sediment (BP E ), and bioavailable P (BP) losses for each event and plot type. Days since Site Plot type Event fertilization SP BP E BP ‐‐‐‐‐‐‐‐‐‐‐‐‐oz P/acre‐‐‐‐‐‐‐‐‐‐‐‐‐ 1 meso 1 ‐‐ 0.58 16.53 17.09 2 1 11.24 5.99 17.23 3 14 4.56 19.56 24.11 4 29 3.03 3.95 6.98 5 49 2.42 1.16 3.60 6 108 0.10 0.28 0.39 21.93 47.47 69.40 2 meso 1 ‐‐ 0.96 7.44 8.39 2 1 15.79 4.72 20.51 3 14 5.33