Short‐ and long‐term legacies of carbon sequestration, and nutrient burial in floodplain wetlands of agricultural and forested catchments, Indiana, United States

Soil organic carbon (C) sequestration and nitrogen (N) and phosphorus (P) burial were measured in two floodplain wetlands' soils of the West Fork of the White River watershed (Indiana, United States) whose catchments differed in land use to better understand how land use practices affect wetland C and nutrient retention. The catchment of one floodplain, Upper West Fork, is dominated by row crop agriculture (61%) whereas the second catchment, Beanblossom Creek, is mostly forested (85%). Soils (0–30 cm) of the two floodplain wetlands had similar bulk density (1.23 g/cm3). Soil organic C and N were low in both floodplains but the percent organic C and N was two times greater (3.3% C, 0.22% N) in the agricultural floodplain than in the floodplain in the forested catchment (1.5% C, 0.14% N). Soil P was three times greater in the agricultural (1100 μg/g) than in the forested floodplain (350 μg/g). Recent soil accretion based on 137Cs which provides a historical record since 1964 (60 years), was two times greater in the agricultural floodplain (2.2 mm/year) than in the forested catchment (1.0 mm/year). Sediment deposition (2500 g/m2/year), C sequestration (90 g/m2/year), and N burial (7.5 g/m2/year) were three times greater in the agricultural floodplain and P burial was seven times greater (3.0 vs. 0.41 g/m2/year). Long‐term measurements (100 years) based on 210Pb did not show large differences in C sequestration and N burial between the two floodplains though soil accretion and sediment deposition were greater in the forested floodplain. We attribute these higher rates to greater erosion in the watershed before 1950 when the catchment had more agricultural land and before instruction on best management practices to reduce soil erosion. These findings confirm previously published studies that show that P enrichment and accumulation in floodplain soils represent legacy effects of agricultural land use in the catchment.


| INTRODUCTION
Wetlands are important sinks for sequestering carbon (C) and removing nutrients, nitrogen (N), and phosphorus (P).Because of the connectivity to aquatic ecosystems, floodplain riparian wetlands are especially important.Floodplain wetlands, with their large areas inundated for long durations, also trap sediment and P that mainly originate from erosion upriver (Noe et al., 2022;Wiener et al., 2022).The deposited sediment subsequently buries substantial amounts of C and N (Craft et al., 2022).
The ability of floodplain wetlands to filter pollutants depends on proximity to sources, especially agricultural land use (Zhang et al., 2023).Floodplains in agricultural watersheds are well suited to intercept large quantities of sediment, N and P because of their proximity to cultivated land (Craft et al., 2018).In contrast, floodplains in predominantly forested catchments are less suited to remove such pollutants (Craft et al., 2022) because they are not intimately connected to the nutrient source(s).
We compared sediment deposition, C sequestration, and N and P burial in two floodplain forests of the agricultural Midwest, also known as the Corn Belt.The floodplain forest of the agricultural catchment, hereafter referred to as the agricultural floodplain, is located on the upper West Fork of the White River.The floodplain of the forested catchment, Beanblossom Creek, hereafter referred to as the forested floodplain, is part of the same West Fork White River watershed.We hypothesize that the floodplain wetland of the agricultural catchment will trap more sediment and remove more P than the floodplain wetland of the forested catchment based on published studies.We further hypothesize that, with greater rates of sediment deposition, the floodplain in the agricultural catchment will bury more N and sequester more C than the floodplain in the forested catchment.

| Site description
We collected soil cores from two floodplain wetlands, Upper West Fork and Beanblossom Creek, within in the West Fork White River watershed in central Indiana (Figure 1).The agricultural floodplain, Upper West Fork, is located north of Indianapolis in Hamilton County.At this point, the river is a third-order stream that drains Killbuck, Pipe, Duck, and Stony Creeks.The geology is primarily glacial outwash derived from Silurian limestone and dolomite with a relatively flat landscape (0%-2% slope).Widespread conversion of forest and prairie to agriculture began in this area from 1850 to 1880 (Quarrier et al., 2022;Thaler et al., 2022).Today row crops, agriculture consisting of mostly corn and soybeans, comprise 61% of the land use in the watershed (Tedesco et al., 2011).
The forested floodplain, Beanblossom Creek, is a third-order stream located in Monroe County (Figure 1).The watershed is made up of ridges with steep slopes (25%-75%) and narrow valleys.The rocks are Mississippian and mostly siltstone rich in silica.Approximately 85%-90% of the watershed upstream of the site is forested (Hoosier Environmental Council, 2008).Both wetlands are inundated for several weeks to months during winter and spring, usually to a depth of 0.5-1 m.
The land use history of the forested catchment (and floodplain) is more complex than that of the agricultural catchment, which has been under intensive cultivation since the late 1800s.Portions of the forested catchment also were farmed beginning in the late 1800s, but because of the hilly terrain and shallow soils compared to the agricultural catchment, much of the land was and still is forested.Before 1950, in areas that were farmed, soil conservation practices such as terracing, contour plowing, conservation tillage (no-till), use of cover crops, and buffer strips were in limited (or no) use, leading to widespread soil erosion (Steiner, 1987;Trimble, 1985) compared to today's agricultural practices.The forested floodplain, Beanblossom Bottoms, also was farmed in the late 1800s but by 1950, farming was abandoned and forest regrowth began.Vegetation in the agricultural floodplain is dominated by an aggrading stand of red maple (Acer rubrum), cottonwood (Populus deltoides), and sycamore (Platanus occidentalis) that are approximately 50 years old.

| Soil sampling and analysis
We collected two 8.5 cm diameter by 30 cm deep soil cores from the agricultural floodplain and two 50 cm cores from the forested floodplain.Cores were sectioned in the field into 2-cm increments and transported to the lab where they were air-dried, weighed for bulk density, ground, and sieved.
Soil from each depth increment was packed into 50 mm diameter × 9 mm deep Petri dishes and analyzed for 137 Cs and 210 Pb.Cesium-137 was measured using gamma analysis of the 661.62 keV photopeak (Craft et al., 2003) to estimate accretion over the past 60 years since 1964-the year of maximum deposition of atmospheric 137 Cs from aboveground nuclear testing (Ritchie & McHenry, 1990).Thus, the 137 Cs maximum or peak in the soil serves as a marker horizon for the year 1964, where the soil surface was located at that time.Total 210 Pb was measured using a gamma analysis of 46.5 keV photopeak (Craft et al., 2003).Excess 210 Pb was calculated using the difference between total and background 210 Pb which was determined from constant 210 Pb from the deeper increments of each core.Accretion rates from 210 Pb were calculated using the constant activity (CA) model (Oldfield & Appleby, 1984) to estimate longterm (approximately 100 years) rates of soil accretion.The CA model is based on a consistent annual input of atmospheric or excess 210 Pb from the decay of Uranium-235 from the earth's crust over time.The CA model describes the exponential decay of 210 Pb, a daughter product of the U-235 decay scheme with age.The model assumes an exponential decrease in 210 Pb with soil depth, leading to monotonic decrease in concentration as seen in Figure 4a,c.The model also assumes a constant accretion rate based on the regression of excess 210 Pb versus depth.
Each depth increment (0-30 cm) also was analyzed for bulk density, organic carbon, total nitrogen, and total phosphorus.Bulk density was calculated from the dry weight per unit volume for each depth increment (Blake & Hartge, 1986).Organic carbon and total N were determined using a Perkin Elmer 2400 CHN analyzer (PerkinElmer).Total P was measured as phosphate in HNO 3 -HClO 4 digestions (Sommers & Nelson, 1972).An internal wetland soils standard yielded a 96% recovery rate for C and N and a 90.5% recovery rate for P. Percent mineral matter was calculated as 100−% organic matter which was assumed to be two times organic C content (Craft et al., 1991).
Mineral sediment deposition, C sequestration, and N and P burial were calculated using 137 Cs-and 210 Pb-derived vertical accretion rates, bulk density, and concentrations down to and including the increment of maximum 137 Cs activity or extent of excess 210 Pb.Mineral sediment deposition was calculated based on accretion, bulk density, and mineral content.

| Soil bulk density and nutrients
Floodplain soils (0-30 cm) were mineral in nature with high bulk density (>1 g/cm 3 ) and low organic C (1%-3%) and N (<0.15%)(Table 1).They varied substantially in P, ranging from 300 μg/g in soils of the forested catchment to 1100 μg/g in soils of the agricultural catchment.
Bulk density, percent C and N, and total P also differed with depth among the two floodplain soils.The forested floodplain had low bulk density (<1 g/cm 3 ) in surface soil (0-5 cm) that quickly increased with depth whereas percent C and N sharply decreased with depth (Figure 2), from 4% to 0.5% for C and from 0.28% to 0.05% for N.In contrast, soils of the agricultural floodplain exhibited a more uniform distribution with T A B L E 1 Mean bulk density, organic C, total N, and total P (0-30 cm) in soils of the two floodplain forests of the White River, Indiana, United States (N = 2 cores per site).
Bulk density (g/cm 3 ) Organic C (%) Total N (%) Total P (μg/g) depth.Bulk density was relatively constant (Figure 2) and consistently greater than 1 g/cm 3 .Soil organic C (2%-3%) did not vary much with depth in the agricultural floodplain either.Soil N, however, exhibited a consistent decrease with depth, from 0.25% at the surface to 0.17% at 30 cm.Soil P in the two floodplains generally decreased with depth though concentrations throughout the 0-30 cm profile were much higher in the agricultural floodplain (Figure 2).

| Soil accretion and accumulation
Soil cores exhibited generally well-defined 137 Cs peaks with the 137 Cs maxima occurring at depths between 5 and 15 cm (Figure 3).Accretion rates based on 137 Cs ranged from 1.0 m/year in the forested floodplain to 3.2 mm/year in soils of the agricultural floodplain.Pb-210 profiles were interpretable in only one of the two cores from each floodplain.In these cores, total 210 Pb exhibited an exponential decrease with depth down to background activities of 4-6 dpm/g (Figure 4).Accretion based on 210 Pb was 2.2 mm/year in the forested floodplain and twice the rate measured using 137 Cs.In the agricultural catchment, 210 Pb accretion was 1.0 mm/year and less than the rate measured using 137 Cs (2.2 mm/year).Deposition of mineral sediment based on 137 Cs ranged from 740 to 3790 g/m 2 /year, with two to five times higher deposition occurring in the agricultural floodplain (Table 2).Carbon sequestration and N burial were three times greater (64-123 g C/m 2 /year, 7.5 g N/m 2 /year) in soils of the agricultural floodplain than in the forested floodplain (27-34 g C/m 2 /year, 2.3 g N/m 2 /year).Nitrogen and P burial also were considerably greater in the agricultural floodplain (Table 2).
Pb-210 sediment deposition, C sequestration, and N and P burial, however, were inconsistent with measurements based on 137 Cs as sediment deposition was greater in the forested floodplain compared to the agricultural catchment (2340 vs. 930 g/m 2 /year, respectively).Furthermore, whereas the agricultural floodplain exhibited higher C sequestration and nutrient burial, 210 Pb-based C sequestration and N and P burial were similar among the two floodplain forests (Table 2).The agricultural floodplain contained higher soil P and greater 137 Cs accretion, sediment deposition, and P burial than the forested floodplain (Tables 1 and 2, Figure 2).Published studies of wetlands in agricultural catchments also exhibit greater soil accretion, elevated soil P and greater C sequestration, and N and P burial relative to wetlands that do not receive agricultural drainage (Craft & Richardson, 1993;Graham et al., 2005) as agricultural catchments export large amounts of sediment and P via eroded soil.The hypothesis of greater sediment deposition, C sequestration, and N and P burial in wetlands with agricultural catchment is thus supported by the findings.
Soil organic C, N, and P also varied with depth among the two catchments.Phosphorus was consistently greater at all depths of the agricultural floodplain (Figure 2).Percent C and N were similar in surface (0-5 cm) soils of the two floodplains, but the agricultural catchment contained greater C and N below this depth.The relatively uniform concentrations of C, N, and P in the upper 30 cm of the agricultural floodplain likely reflect the history of row crop agriculture and its ploughing that homogenizes the surface soil while the high soil P concentration also reflects the legacy effect of P inputs from long-term agriculture in the catchment.In contrast, percent C and N in the floodplain of the forested catchment were greatest at the surface and declined strongly with depth, indicating the reforestation of the floodplain after agriculture was abandoned on site in the mid-20th century (discussed in detail below).
In contrast to 137 Cs, which showed consistently greater soil accretion, sediment deposition, and C, N, and P accumulation in the agricultural floodplain, measurements based on 210 Pb did not.Except for P burial, which was marginally greater in the agricultural floodplain, soil accretion, and sedimentation were double in the forested floodplain (Table 2).Pb-210 C sequestration and N burial did not strongly differ between the agricultural and forested floodplains despite accretion rates that were double (Table 2).The similarity in 210 Pb-based C sequestration and N burial among the two catchments was the result of consistently greater percent C and N with depth in the agricultural floodplain relative to the forested floodplain where percent C and N strongly decreased with depth (Figure 2).
Greater 210 Pb accretion and sedimentation in the forested floodplain likely reflect agricultural activities in the catchment that occurred in the past, before about 1950.The Beanblossom Creek floodplain was cleared and tilled beginning in the 1800s and continued into the 20th century (https://members.wetlandsofdistinction.org/woddirectory/ Details/beanblossom-bottoms-nature-preserve-2016983).After the Second World War, cultivation ceased and resulting reforestation of the floodplain has led to a 75 year-old hardwood forest that exists today (2024).In contrast to 137 Cs which measures soil accretion from the past 60 years since 1964, 210 Pb measurements describe conditions over the past 100-125 years, from about 1900 onward.In the agricultural Midwest, the first half of the 20th century was characterized by limited soil conservation practices such as terracing, contour plowing, conservation tillage (no-till), use of cover crops, buffer strips, and others (Steiner, 1987;Trimble, 1985).The conversion of native prairie and forest vegetation plus the lack of conservation practices led to severe soil erosion beginning in the second half of the 18th century and continued into the 20th century (Quarrier et al., 2022;Thaler et al., 2022).Furthermore, fertilizer application was quite limited until the Agricultural catchment 1.0 1.0 42 2.9 1.12 930 Forested catchment 2.2 45 3.9 0.86 2350 advent of widespread production of inorganic N and P fertilizer after the Second World War (Cao et al., 2018;Cordell & White, 2014).It is likely that the higher rates of 210 Pb soil accretion and sediment deposition in the forested floodplain in the past are also a result of the steeper sloped landscape (25%-75%) in addition to the lack of conservation tillage practices.Overall the steep slopes historically provided a rich source of eroded sediment to the wetland.In contrast, the catchment of agricultural wetland lies on the till plain of the Wisconsonian glacier of the Ice Age 20,000 years ago (Hoosier Environmental Council, 2008) which, today remains a nearly flat (0%-2%), featureless landscape conducive for intense row crop agriculture if given adequate drainage.
Other published studies of floodplain C sequestration, N burial, and sediment deposition within the Midwest highlight the legacy of land usage on wetlands.Mean rates originate mostly from the United States and are based on 137 Cs and 210 Pb that are quite similar to each other unlike this study (Table 3).Cs-137 P burial was somewhat greater (0.91 g/m 2 /year) than 210 Pb (0.70 g/m 2 /year) which may be due to the increasing use of P fertilizers worldwide since the mid-20th century.N and P burial were greatest in a Wisconsin floodplain (2.6 g/m 2 /year) whose catchment was primarily in agriculture (Johnston et al., 1984).These rates were substantially greater than those measured in our study and others.Carbon sequestration was greatest in floodplains of Maryland, United States and Costa Rica (Table 3).Sediment deposition was also greatest in the Costa Rican floodplain.
There is much interest in soil C sequestration and the high rates of recent ( 137 Cs) C sequestration in the floodplain of the agricultural catchment relative to the floodplain of the forested catchment suggests that sediment transport from farmed uplands promotes its burial.How much of the sequestered C is produced in situ versus transported by periodic flooding is unknown.Overall, catchment-scale C sequestration by floodplain wetlands depends mostly on the amount of floodplain habitat within the catchment, which generally is small since historically many were drained and converted to agriculture.Kleiss (1996).e Kroes and Hupp (2010).f Johnston et al. (1984).Both catchment and wetland land use history produce legacies associated with long-and short-term agriculture.Recent ( 137 Cs, since 1964) rates of sediment deposition and C, N, and P burial were three to six times greater in the agricultural floodplain.Soil P enrichment and greater P burial in the agricultural floodplain is associated with the legacies of widespread fertilizer in this predominantly agricultural (61%) catchment since the mid-20th century while at the same time, the forested floodplain was an aggrading forest in a predominantly forested (85%-90%) catchment.However, long-term ( 210 Pb) rates revealed evidence of agriculture (erosion and sediment deposition) in the forested floodplain pre-1950, when the floodplain and the catchment were tilled.
Disentangling the legacy effects of agriculture, including erosion and sediment deposition, fertilizer history, and implementation of conservation practices are key to explaining historical patterns of sediment, carbon, and nutrient retention in floodplain wetlands of the agricultural Midwest.

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I G U R E 2 Change in (a) bulk density, (b) organic C, (c) total P, and (d) total N with depth in soil cores collected from the agricultural and forested floodplains.

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I G U R E 3 Depth distribution of 137 C in soil cores collected from soils of (a, b) agriculture land and (c, d) forested floodplains.

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I G U R E 4 Depth distribution of total and excess 210 Pb in soil cores collected from soils of (a, b) agricultural and (c, d) forested floodplains.T A B L E 2 Soil accretion (mm/year) and sediment, organic C, N, and P accumulation (g/m 2 /year) in two floodplain forests of the White River, Indiana, United States.

g
Bernal and Mitsch (2012) based on 137 Cs and 210 Pb.

k
Bernal and Mitsch (2013) based on 137 Cs and 210 Pb.
Carbon sequestration, nitrogen and phosphorus burial, and sediment deposition in floodplain soils based on 137 Cs (first number) and 210 Pb (second number).