Intermittent agitation of liquid manure: effects on methane, microbial activity, and temperature in a farm-scale study

Figures & data

Table 1. Manure characteristics: Dry Matter (DM), Volatile Solids (VS), Organic Carbon (OC), pH, Electrical Conductivity (EC), Nitrogen (N), Ammonia (NH₃), Phosphorus (P), Potassium (K), total Volatile Fatty Acids (VFA), and Acetic Acid (AA) before, during, and after each agitation event (AG1-AG3), and on average.

	AG1			AG2			AG3			Average
Parameter	Before	During	After	Before	During	After	Before	During	After	Before	During	After
DM (%)	4.14	4.86	4.06	3.74	4.47	3.98	3.49	4.23	3.99	3.79	4.52*	4.01
VS (%)	2.96	3.66	2.88	2.59	3.19	2.73	2.45	3.04	2.84	2.67	3.30*	2.82
OC (%)	1.48	1.83	1.44	1.3	1.6	1.37	1.23	1.52	1.42	1.34	1.65*	1.41
pH	7.33	7.27	7.4	7.3	7.45	7.59	7.32	7.26	7.28	7.32	7.33	7.42
EC (mmhos cm^−1)	18.7	14.4	17.17	17.5	17.3	17.7	15.8	15.1	15.5	17.3	15.6	16.8
N (%)	0.55	0.33	0.3	0.42	0.27	0.32	0.44	0.27	0.32	0.47	0.29*	0.31*
NH3 (ppm)	1750	1,500	1200	1250	1200	1260	1170	1770	1560	1390	1490	1340
P (%)	0.06	0.06	0.06	0.05	0.06	0.06	0.05	0.06	0.05	0.05	0.06	0.06
K (%)	0.25	0.25	0.25	0.24	0.26	0.27	0.24	0.25	0.24	0.24	0.25	0.25
VFA (mg L^−1)	7,017	10,356	5,060	2,412	2,133	1,232	85.7	179.7	370.8	3172	4223	2221
AA (mg L^−1)	191.1	298.4	248.2	168.5	187.2	221.4	76.3	171.1	206.2	145	219	225*

*indicates a significant difference between During vs Before agitation, and After vs Before agitation.

Figure 1. Weekly average quantity of manure in storage during 2015 and 2016. Top panel: depth (m) in the small tank (314 m²; dashed lines) and large tank (1257 m²; solid lines). Bottom panel: total volume (m³) of stored manure in both tanks combined.

Table 2. Weekly average depth in the large tank (m), air temperature (°C, T_a), and stratified manure temperatures (°C, T_m) for weeks 30–38 in 2015 and 2016.

	2015					2016
	(m)	(°C)				(m)	(°C)
Week	Depth	Ta	Tm	Tm	Tm	Depth	Ta	Tm	Tm	Tm
			top	middle	bottom			top	middle	bottom
26	1.7	19.8	ND	ND	ND	1.4	21.4	23.2	13.9	8.4
27	1.6	17.6	ND	ND	ND	1.5	20.4	22.3	16.9	11.2
28	1.8	20.7	ND	ND	ND	1.7	22.5	22.1	17.3	13.9
29	1.9	20.1	ND	ND	ND	1.9	20.6	23.7	22.4	15.4
30	2.0	20.1	22.4	21.2	15.2	2.0	21.8	24.2	23.3	16.4
31	2.3	23.0	22.8	22.7	17.4	2.2	20.6	25.0	24.7	18.3
32	2.0	18.2	22.5	22.4	20.1	2.3	23.0	25.9	25.0	20.0
33	1.7	19.7	21.9	21.9	19.7	2.4	22.4	25.9	25.1	20.1
34	1.9	22.8	23.3	22.6	20.4	2.2	21.1	25.6	24.9	20.1
35	2.0	17.2	20.2	22.1	20.6	2.2	20.8	25.2	24.4	20.2
36	2.1	20.6	21.5	21.6	20.2	2.3	18.7	25.0	23.9	20.0
37	1.9	20.1	21.5	22.1	20.4	2.3	20.4	24.7	24.0	19.6
38	1.1	19.1	20.6	21.7	20.3	2.0	15.4	23.2	22.8	19.0
Avg. (30–38)	1.9	20.1	21.9*	22.0*	19.4	2.2	20.5	25.0*	24.2*	19.3

Note. Manure temperature was measured 15-cm below the surface (top), 1-m below the surface (middle), and 10 cm above the bottom (bottom). ND indicates no data available. Manure depth increased due to continual transfer of fresh manure from the barn into the storage, and manure depth decreased periodically due to transfers from this tank to a secondary storage tank.

*Significantly different between years at p < 0.05.

Figure 2. Weekly time series (weeks 24–44) of: air temperature (°C, top panel), manure temperature at 10 cm depth (°C, 2^nd panel), manure temperature at mid-depth (°C, 3^rd panel), and manure temperature at the bottom of the large storage tank (°C, bottom panel).

Figure 3. Stratified (top, middle, bottom) manure temperature (T_m, °C) profile of the large storage tank before and after agitation events. Top panel: Agitation 1 (AG1), DOY 196; middle panel: Agitation 2 (AG2), DOY 208; bottom panel: Agitation 3 (AG3), DOY 245. Each panel shows approximately 3d of data before and after each agitation event. Note: Gap between pre- and post- agitation data segments is not to scale.

Table 3. Summary of qPCR (gene copies and transcript copies) targeting Bacterial 16S rRNA (first), Archaeal 16S rRNA (second), and mcrA (third) from samples taken before, during and after three agitation events (AG1, AG2, AG3).

	Abundance (Gene Copies per g dry manure)			Activity (Transcript Copies per g dry manure)
	AG1	AG2	AG3	AG1	AG2	AG3
	Bacteria (16S rRNA gene)
Before	3.6E+11^a	5.5E+11^a	5.8E+11^a	2.2E+12^a	1.4E+12^a	4.8E+12^a
During	5.5E+11^b	5.3E+11^ab	5.8E+11^ab	1.1E+12^ab	1.4E+12^a	3.3E+12^a
After	7.0E+11^b	6.9E+11^b	7.8E+11^b	3.0E+12^b	5.0E+12^b	4.7E+12^a
	Archaea (16S rRNA gene)
Before	7.9E+10^a	1.7E+11^a	2.3E+11^a	2.8E+08^a	1.4E+09^a	1.6E+11^a
During	1.0E+11^a	1.8E+11^ab	3.4E+11^b	3.8E+07^a	6.8E+08^a	1.2E+11^a
After	1.5E+11^b	2.5E+11^b	2.9E+11^ab	1.6E+09^b	1.0E+11^b	2.9E+11^a
	Methanogens (mcrA gene)
Before	2.1E+10^a	1.8E+10^a	2.2E+10^a	4.3E+05^a	2.2E+06^a	7.7E+06^a
During	2.0E+10^a	1.5E+10^a	2.1E+10^a	4.0E+05^a	8.8E+06^ab	5.2E+06^ab
After	3.0E+10^b	2.2E+10^b	2.5E+10^a	5.3E+05^a	1.1E+07^b	1.3E+07^b

Note. Superscript letters indicate differences between levels of each parameter before, during, and after each agitation (i.e. differences between rows within each column).

Table 4. Summary of methane (CH₄) emission for 2014, 2015, and 2016 during the main period of methane emissions, which occur between spring and fall manure removal.

	Weeks 18–44			Weeks 18–37
	2014	2015	2016	2014	2015	2016
Cumulative CH4 emissions (kg)	38,863	43,871	56,108	24,073	38,996	41,642
Number of days	189	189	189	140	140	140
Average CH4 emissions (kg d^−1)	206	232	297	172	279	297
Volume of manure stored* (m^3)	NA	5,394	5,090	NA	4,218	3,914
Mass of VS stored (Mg)	NA	135.2	149.5	NA	105.7	115.0
CH4 scaled by volume (kg m^−3)	NA	8.1	11.0	NA	9.2	10.6
CH4 scaled by VS (g kg^−1)	NA	325	375	NA	369	362
CH4 scaled by VS (g kg^−1 d^−1)	NA	1.7	2.0	NA	2.6	2.6

Note. Due to the differing timing of fall removal, average and cumulative emissions were calculated for the time period where tank depth was similar for both years (weeks 18–37), and for the entire monitoring period (weeks 18–44).

*volume of manure added to the storage during the monitoring period + initial volume on week 18

Figure 4. Scaled methane (CH₄) emissions (g m⁻³ d⁻¹) from week 15 to week 44 of 2015 and 2016.

Figure 5. Daily time-lapse series showing surface foam formation of the large storage tank the day before first agitation event [AG1 (−1d)], during the first 5 hours of agitation (AG1 0h and 5h), the day after agitation [(AG1 (+1d)], until 5 days after agitation [AG1 (+5)]. Except during agitation, pictures were captured between 06:30 a.m. and 07:00 a.m. (before foam dissipated in the late morning).