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

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

Grazing Management StrategiesLow stocking density Ultra high stocking density10 steers for 27 days in both pastures = same stocking rate

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

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

Project Scope cont…

Plant pool

CowPool

ExcretaPool

Soilpool

Spatially coupled nutrient cycling

Plant pool

CowPool

ExcretaPool

Soilpool

Spatially un-coupled nutrient cycling

Project Objectives

a) Size of pulsesa) Dungb) Litterc) Urine

b) Pattern of returnc) 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 fatef) Pasture level estimations of nutrient cyling

UpscalingFate of dung C &N

Two MSc. students

MSc. student

PhD student

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

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

Litter Deposition Rate

• Higher % trampling in MOB• No difference in litter deposition• Highest litter deposition duringgrazing season

Miles Redden, MSc. thesis, UNL, 2014

Trampled plant biomass

Low stocking

High stocking

Dung beetle frequency and diversity

• Higher dung beetle diversity in rotational grazing• 22 different dung beetle species across ranch sites

High stockingLow stocking 4PR1Low stocking 4PR2ContinuousHay

Patrick Wagner, MSc. thesis, UNL, 2016

Fate of dung C & NQuantify 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)

• 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

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 SeasonNo 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 SeasonNo Beetle

Beetle

Control

Kenneth Evans, MSc. thesis, UNL, 2016

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)

Spatial & temporal distributionMSc. 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

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

Accomplishments Postdoc Graduate*

studentsUndergraduate students

Field/labsupport staff

1 3 MSc1 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 placementin 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