Incorporation of fallow weed increases phosphorus availability in a farmer’s organic rice fields on allophanic Andosol in eastern Japan

Figures & data

Figure 1. Total P concentration in the fields under organic farming for various number of years with (filled circles) and without guano (unfilled circles) application.

Figure 2. Relationships between the number of years under organic farming and the ratio of Truog P (Tru-P) to total P (A) and that of Bray-2 P (Asc-P) to total P (B) with (filled circles) and without guano (unfilled circles) application. The dotted curves show the quadratic regression of the respective ratios on the number of years under organic farming.

Figure 3. Relationship between the number of years under organic farming and the amount of P in Foxtail, the dominant weed species. Circles are observations, and the curve is the model fitted to the observations.

Table 1. Effect of suspension of weed incorporation in paddy fields on soil Tru-P and Asc-P. (mg P₂O₅ kg⁻¹).

			Tru-P^c		Asc-P
Years of organic farming	Field ID^d	Treatment^a		Difference between CWI and SWI		Difference between CWI and SWI	Change after treatment
18		Before^b	NA		4140
	8	CWI	626		4233		92
		SWI	605	21.0	4013	219.5	−127
		Before	NA		4530
	9	CWI	731		4735		205
		SWI	690	41.3	4379	356.6	−151
		Before	NA		4295
	10	CWI	719		4606		311
		SWI	689	30.1	4263	342.9	−32
10	51	CWI	449		2985
		SWI	367	81.7	2515	470.5	NA
	52	CWI	679		4403
		SWI	656	22.9	4316	86.5	NA
	53	CWI	649		4320
		SWI	636	12.9	4089	230.9	NA
	Mean of the fields	CWI	642		4214
		SWI	607	35.0	3929	284.5

^aTreatment was conducted in 2014 and 2015 rice seasons.

^bSoil sampling was done before treatment in October 2013, and soil sampling was done after treatment in October 2015. CWI: continuous weed incorporation, SWI: suspended weed incorporation.

^cTru-P denotes Truog-P. NA denotes not available. Asc-P denotes Bray-2-P after reduction by ascorbic acid.

^dHistory of the organic farming practices in the respective fields can be found in Supplementary Table 1.

Table 2. Increase of available P relative to P inputs due to the continued weed incorporation as compared with the plots with suspended weed incorporation for 2 years (2014–2015).

Field ID	Biomass of weeds		P content of weeds		P input from weeds for 2 years	Increase of available P^a		Ratio of available P increase to P input from weeds
	2014	2015	2014	2015		Tru-P^b	Asc-P^c
	g m^−2		%P2O5		mg P2O5 m^−2	mg P2O5 m^−2		Tru-P	Asc-P
8	275	518	0.65	0.63	5051	2103	21951	0.4	4.3
9	323	472	0.65	0.62	5011	4130	35655	0.8	7.1
10	383	434	0.66	0.75	5755	3013	34288	0.5	6.0
51	809	474	0.55	0.56	7154	8169	47045	1.1	6.6
52	502	446	0.61	0.66	6014	2294	8646	0.4	1.4
53	483	463	0.60	0.59	5621	1288	23091	0.2	4.1
MEAN	463	468	0.62	0.63	5768	3500	28446	0.6	4.9
SD	191	29	0.04	0.06	786	2479	13379	0.3	2.1

^aIncrease of available P on area basis was calculated from the difference in Av-P concentrations between the plots with continued weed incorporation and those in suspended weed incorporation plot on the assumptions of 15 cm plow layer soil depth and bulk density of 0.67.

^bTru-P denotes Truog-P.

^cAsc-P denotes Bray-2-P after reduction by ascorbic acid.