This document provides an overview of a research project studying the effects of different grazing management strategies on carbon and nitrogen cycling in rangelands. The project compares a low stocking density grazing strategy to an ultra-high stocking density strategy. Key findings include greater vegetation trampling but similar litter deposition under high density grazing. The project also found higher dung beetle diversity under rotational grazing regardless of stocking density. Dung beetle activity did not significantly impact greenhouse gas fluxes or dung decomposition rates. Spatial and temporal patterns of dung distribution will be modeled to estimate ranch-level nutrient cycling under different management strategies.

1. Grazing Management Effect on Micro- and
Macro-Scale Fate of C and N in Rangelands
M. Mamo, J. Bradshaw, K. Evans**, K. Eskridge, R.
Ferguson, J. Guretzky, K Jenkins, A. Sanford**,
W. Schacht, J. Volesky, P. Wagner**, A. Wingeyer, H. Yang

2. Outline
• Background
• Project Scope
• Objectives
• Site Description
• Result-highlights
• Summary
• Accomplishments

3. Grazing Management Strategies
Low stocking density Ultra high stocking density
10 steers for 27 days in both pastures = same stocking rate

4. Proposed advantages of ultra high stocking
density (MOB)
• Increased SOM
• Increased pasture productivity
• Increased species diversity
• Increased dung beetle activities
• Uniform fecal & urine deposition
• Increased animal production per area

5. Project Scope
Questions
• Are the rates of decomposition and/or
incorporation of nutrient pulses under specific
grazing strategies regulated by the spatial and
temporal distribution of these pulses?
• How do these affect nutrient cycling and
nutrient use efficiency?
• What are the levels of association of grazing
strategies to pattern of nutrient return and soil
nutrient availability

6. Project Scope cont…
Plant
pool
Cow
Pool
Excreta
Pool
Soil
pool
Spatially coupled nutrient cycling
Plant
pool
Cow
Pool
Excreta
Pool
Soil
pool
Spatially un-coupled nutrient cycling

7. Project Objectives
a) Size of pulses
a) Dung
b) Litter
c) Urine
b) Pattern of return
c) Pattern of dung
beetles
C &N pulses in relation to grazing strategies
d) Decomposition,
transport &
transformations of
C & N
e) Relationships of
grazing/pulses/beetles/
nutrient fate
f) Pasture level
estimations of nutrient
cyling
Upscaling
Fate of dung C &N
Two MSc. students
MSc. student
PhD student

8. Site Description
• The Barta Brothers Ranch
– 2100 ha in Eastern Sandhills
• 2000 ha of upland range
• 100 ha of meadows
– 560 mm annual precipitation
– 75% during growing season
• Other collaborating ranches

9. Experiment Design
• Grazing: May – August (60-80 d)
• Stocking Rate:7.5 animal unit
months ha-1
• Low stocking density:
• 7000 kg ha-1
• 10-15 d pasture-1
• Ultra high stocking density:
• 225,000 kg ha-1
• 0.5 d pasture-1

10. Litter Deposition Rate
• Higher % trampling in MOB
• No difference in litter deposition
• Highest litter deposition during
grazing season
Miles Redden, MSc. thesis, UNL, 2014
Trampled plant biomass
Low stocking
High stocking

11. Dung beetle frequency and diversity
• Higher dung beetle diversity in rotational grazing
• 22 different dung beetle species across ranch sites
High stocking
Low stocking 4PR1
Low stocking 4PR2
Continuous
Hay
Patrick Wagner, MSc. thesis, UNL, 2016

12. Fate of dung C & N
Quantify the fate of nutrients from decomposing
dung pats with and without dung beetles
– Two treatment:
• No beetle (i.e., with mesh)
• With beetle (i.e., no mesh)
– Two seasonal exps each in 2014 & 2015
• June
• July
• Soil and GHG sampling times: 1, 3, 7, 14, 28 and 56 d.
(Bertone et al., 2006)

13. • 4 Seasonal Experiments with 8 Rep
– Exposed dung pats
– Unexposed dung pats with 1 mm mesh exclosure
– No dung pat
Unexposed
No dung
Exposed
Fate of dung C & N

14. CO2 Flux
0
500
1000
1500
2000
2500
Exposed Unexposed No Dung
CO2(g∙m‐2)
Treatment
Cumulative Flux
• Greater flux from exposed compared to
unexposed treatments observed only in 1 out
of 4 experiments
• Greater flux from dung compared to no dung
in 3 out of four seasonal experiments
• VWC, and [O2], significant across all seasonal
experiments
0
10
20
30
40
50
60
0 10 20 30 40 50 60
CO2 Flux (g∙m‐2∙d‐1)
Days After Dung Placement
Early Season
No Beetle
Beetle
Control
0
10
20
30
40
50
60
0 10 20 30 40 50 60
CO2 Flux (g∙m‐2∙d‐1)
Days After Dung Placement
Late Season
No Beetle
Beetle
Control
Kenneth Evans, MSc. thesis, UNL, 2016

15. Dung Decomposition
wet dryDung moisture
Hypothesis:
Speed of dung decomposition
200
220
240
260
280
300
320
340
0 10 20 30 40 50 60
Dry Matter (g)
Days After Dung Pat Placement
No Beetle
Beetle
S=0.57
S=0.62
• No differences in DM loss
• Speed of aging (S) similar
DK C&N model (Yang & Janssen, 2000)

16. Spatial & temporal distribution
MSc. student (Amanda
Sanford)
• Dung pat distribution in
time & space
• Dung spectral
characteristics
• Dung C&N estimation
Phd. student (modelling)
• Integration
• Relationships of
management to pulses

17. Summary
• Greater vegetation trampling in high density grazing,
however,
– No different in litter deposition
– Biomass production
• High diversity of beetle in rotation grazing regardless
of stocking density
• Dung beetle effect was not significant on GHG fluxes
and dung decomposition
• Spatial and temporal patterns of dung distribution will
be used to estimate ranch level nutrient cycling-
integrating field observations and process models

18. Accomplishments
Postdoc Graduate*
students
Undergraduate
students
Field/lab
support
staff
1 3 MSc
1 PhD
(2
females/2
males)
10 (5 females/5
males)
4
• Manuscripts in Prep.: 4
• Manuscripts planned: 3
• Symposium ASA-SSSA-CSA-ESA 2015
• Field day 2015
• Stakeholder workshop- 2014
• Evans & Wagner (MSc)-high placement
in graduate student poster competition
This project is supported by Agriculture and Food Research Initiative Competitive Grant
Program no. 2013-67019-21394 from the USDA National Institute of Food and Agriculture