Comparison of Soils and Plants at Prairie Ridge: % C and % N Lori Skidmore

Plant - Soil Interactions Knops and Tilman (2000) conducted a longterm study of an abandoned agricultural site. –Rates of C and N accumulation in soil significantly influenced by plant composition –C3 grasses and forbs  decreased C and N accumulation rates –C4 grasses  increased the C:N ratio of soil organic matter  increased C accumulation rate  did NOT increase N accumulation rate

Hypothesis Soil samples collected near C3 grass (fescue) and forbs (ragweed and horsenettle) will have a lower % C than soils collected near C4 grass (bermuda). %N will be similar in surface soils throughout the plot. Plant fragments in soils will reflect % C and % N of dominant plant species (bermuda grass) in sample plot. Do we see this?

PRP-9 fescue PRP-3 ragweed PRP-4 ragweed PRS-5 surface soil PRP-6 bermuda PRS-7 surface soil PRP-11 bermuda PRP-12 fescue PRP-8 horsenettle PRP-13 (dead plant matter) PRP-14 bermuda Soil Pit PRS -15 (+2 – 0 cm) PRS -16 (0 – 6 cm bgs) PRS -17 (6 – 11 cm bgs) PRS -18 ( + 11 cm bgs) Not to scale N Sample Plot

Plant and Soil Data Calculated average values for replicate analyses of soils and plants. –Excluded questionable analyses. Calculated “whole soil” compositions from bulk soil and soil plant fragment analyses: whole soil % C = (bulk soil wt * bulk soil %C) + (plant wt * plant %C) bulk soil wt + plant wt whole soil % N = (bulk soil wt * bulk soil %N) + (plant wt * plant %N) bulk soil wt + plant wt

“Corrected” Soil Compositions Correction for plant fragments picked out changed whole soil composition little. Plant fragments were only 0.03 – 1.16 % of total sample weight.

Average Soil % C Bulk soil range was % C Plant fragments range was – % C Whole soil range was 0.75 – 4.16 % C

Average Soil % N Bulk soil range was 0.05 – 0.36 % N Plant fragments range was 0.56 – 1.93 % N Whole soil range was 0.05 – 0.36 % N

ROOT LEAF TOTAL Transect – NE side of plot NOT TO SCALE Horsenettle (C3) Ragweed (C3) Bermuda (C4) Fescue (C3) %C 1.39 %N %C 1.79 %N %C 3.08 %N %C 1.73 %N ForbsGrasses %C 1.51 %N %C 2.59 %N %C 2.57 %N %C 2.18 %N %C 1.44 %N %C 1.54 %N %C 2.17 %N %C 0.83 %N All soils: 2.32 %C, 0.19 %N PRS-15: 1.90 %C, 0.14 %N PRS-16: 2.35 %C, 0.19 %N PRS-17: 0.82 %C, 0.06 %N PRS-18: 0.75 %C, 0.05 %N PRS %C 0.36 %N PRS %C 0.36 %N

Fescue C3 Ragweed C3 Ragweed C3 PRS %C 0.36 %N Bermuda C4 PRS %C 0.36 %N Bermuda C4 Fescue C3 Horsenettle C3 Bermuda C4 PRS-15: 1.90 %C, 0.14 %N PRS-16: 2.35 %C, 0.19 %N PRS-17: 0.82 %C, 0.06 %N PRS-18: 0.75 %C, 0.05 %N Not to scale N Sample Plot

Conclusions Highest %C and %N in surface soil where bermuda (C4 grass) was dominant Lower %C and %N in surface soil near forbs (C3) Lowest %C and %N in soil “pit” beneath large bunch of fescue (C3 grass) %C pattern consistent with hypothesis, but %N is not

My issues with this data… Sample depths Sample distribution in plot Soil treatments were not the same Did not get a good density separation Did not perform grain size analysis –C and N concentrations may be related to percent clay content (Hughes et al., 2002)

References Knops, J.M.H., and D. Tilman Dynamics of soil nitrogen and carbon accumulation for 61 years after agricultural abandonment. Ecology, Vol. 81 (1), pp Hughes, R.F., J.B. Knuffman, and D.L. Cummings Dynamics of aboveground and soil carbon and nitrogen stocks and cycling of available nitrogen along a land-use gradient in Rondonia, Brazil. Ecosystems, Vol. 5, pp